As John Lawler already said in the comments, there are two clauses there: the protasis proved, and the apodosis, *These allegations would be a wonderful indictment to the club. The protasis has the form of a past-participial, and in constructions like that we normally take such a past-participial to be a non-finite, subjectless clause.
While your question was not about the apodosis, I should nevetheless mention for completeness that there is a problem with it (as others have already commented, and which is why I had to put a '*' in front of it). Namely, you can't say an indictment to the club; it must be of the club. The noun indictment does not licence a modifier in the form of a preposition phrase (PP) headed by to. Thus, either you need to change to to of, or change indictment to a noun that does allow such a modifier, e.g. blow or, as John Lawler suggested, introduction. The word wonderful is also a bit odd here, though I can imagine contexts where it could work.
I will proceed as follows.
First, I will show that proved in your sentence is a past-participle, as opposed to either an adjective or a preterite form of the verb. Once this is established, it will follow that the corresponding 'full form' (or, 'canonical' form) of the if-clause would be if they are proved. Here the protasis they are proved is a passive construction.
Clearly, the canonical form they are proved is a clause. What about the 'reduced' form, which is just the past-participle proved? As I will then explain next, such a past-participle is indeed considered a non-finite clause, as opposed to just a verb phrase.
Proved is a past-participle here
First of all, let's convince ourselves that proved is a past-participle in your sentence, and not an adjective. One way to disambiguate between adjectives and past-participles is by what kinds of complements they can take. In particular, only verbs can take a predicative complement (CGEL, p. 79), and it is clear that proved can indeed take such a complement: These allegations, if proved true/a fact, would be… (Note that CGEL (p. 264) confirms that prove in its transitive sense can take a predicative complement, e.g. in We proved it genuine/a fake.)
Second, let's establish that proved is a past-participle as opposed to a preterite. We need to do this because proved can be either the preterite form or the past participle form of the verb prove; this step would have been unnecessary if the form proven had been used. If proved were a preterite form here, it would follow that the full form of the conditional adjunct would be e.g. if he proved them. But that would mean that both the subject and the object were ellipted, and the ellipsis of the subject is very unlikely here, for two reasons. First, in English, the subject can normally be missing only in imperatives and non-finite clauses (CGEL, p. 236), but here we would have a canonical declarative clause he proved them, with the finite verb proved, from which we ellipted both the subject and the object. Second, ellipted elements must be recoverable from context (CGEL, p. 1193). But in the case of your sentence, it is as likely as not that the context will not explain who, exactly, is supposed to be doing the proving; it is precisely in such cases, where the subject is irrelevant or unknown, where we normally resort to the passive construction (they are proved/proven). We conclude that the full form of the conditional complement is the passive if they are proved, and not the canonical if he/Mary/someone/… proved it.
CGEL confirms the above analysis in the following section (pp. 756-757), where the relevant example is [i]:
14.4 Other explicitly or implicitly conditional constructions
(a) Reduction of the complement of the conditional preposition
We have been concerned so far with constructions where if (or unless) has a full content clause as complement. Other possibilities are illustrated in:
 i This product will/would stay fresh for two weeks, if kept refrigerated.
ii There'll probably be a vacancy in June; if so, we'll let you know.
iii We may be able to finish tomorrow; if not it will certainly be done by Friday.
iv You wont get your money till next month, if then.
v Some, if not all, of your colleagues will disagree with that view.
vi We'll get it finished by tomorrow if necessary/possible.
In [i] the protasis has the form of a past-participial. This is found in both open and remote constructions: the full versions would be if it is kept refrigerated and if it were kept refrigerated. Past-participials are found also with unless: Do not take any further action unless requested to do so. In [ii-iii] the protasis is a pro-form standing for a clause (cf. Ch. 17, §7.7.2) with so positive and not its negative counterpart. If so is interpreted as "if there is a vacancy in June", if not as "if we aren't able to finish tomorrow". Examples [iv-v] illustrate the construction where an if phrase serves to cancel an implicature of the apodosis. On its own, you wont get your money till next month, for example, implicates that you will get your money next month: if then indicates that you may not get it even then. In [vi] the protasis is an AdjP (adjective phrase) headed by one or other of the modal adjectives necessary and possible. They can be modified (if absolutely necessary, if at all possible), but not replaced by other adjectives (*if useful). The understood clausal subject can be reconstructed from the apodosis: "if getting it finished by tomorrow is necessary/possible".
The past-participle is a non-finite clause, and not just a verb phrase
Here is the relevant section of CGEL (p. 1175):
The great majority of non-finite clauses have no subject, as in:
 i Kim was glad [__to reach home].
ii It has been a pleasure [__meeting you].
iii Anyone [__living nearby] will be evacuated.
iv The sum [__spent on gambling] was extraordinary.
Whereas the subject is an obligatory element in canonical clauses, there are no non-finite constructions in which a subject is required. (The subordinator for cannot occur without a subject, but we do not find constructions where for + subject is required.) There are, moreover, many constructions where it is impossible to add a subject, as in [iii—iv], or the examples of  above.
It will be evident from this formulation that we take the subject to be an optional element in non-finite clauses, not an element whose presence is necessary for an expression to qualify as a clause. That is, the bracketed expressions in  are analysed as clauses that consist of just a VP rather than simply as subclausal expressions with the form of a VP. We take the VP to be the head of the clause and the presence of a VP is normally sufficient to establish clausal status. The main exception is with attributive VPs in NP structure:
 i our rapidly approaching deadline
ii a poorly drafted report [VPs, not clauses]
Expansion of the verbs in this construction is virtually limited to adverbial modifiers preceding the verb: the range of structural possibilities here is quite different from that found in clauses.
A note on terminology
Consider the sentence
 If you touch that wire, you will get an electric shock.
CGEL (p. 738) takes If you touch that wire to be a preposition phrase (PP), whose head is the preposition if and whose complement is a content clause (you touch that wire). Such a PP is called a conditional adjunct. The subordinate clause functioning as complement of if is called the protasis. The rest of the sentence (you will get an electric shock) is called the apodosis. The full matrix clause  is called a conditional construction. Thus, conditional construction = conditional adjunct + apodosis, where conditional adjunct = if + protasis.
CGEL notes the following:
Traditional grammar takes if to be a subordinating conjunction, not a preposition, and many modern works follow this analysis; if is therefore commonly regarded as forming part of the protasis. We are using 'conditional adjunct' for the constituent including if and protasis just for the subordinate clause. Protasis and apodosis derive from Greek; it may be helpful to note that the pro • prefix means "before" (as also in prologue, etc.): the protasis is logically prior to the apodosis. In logic the terms 'antecedent' and 'consequent' generally correspond to protasis and apodosis respectively, but we will see that the correspondence is not complete. (This sense of 'antecedent' is quite different from the grammatical one, but the prefix ante • , this time Latin, again means "before".) Note also that we use 'conditional construction' rather than 'conditional clause', because the latter could be understood as applying to either the subordinate clause or the superordinate one.
Note that CGEL does take if to be a subordinator in those constructions in which it is equivalent to whether, as in Ask him if he minds (p. 600).