This usage is not only acceptable, it is standard, although not exclusive. The subject, for its essential, is treated in § 7.22 from A comprehensive grammar of the English language (Quirk et al.). It is a usage that the adverb "too" and "enough" also share.
(CoGEL) An adjective modified by enough, too, or so can be separated from its
complementation if the modified adjective is placed before the indefinite (or
zero) article of the noun phrase [8, 9]:
♦ She is brave enough a student to attempt the course. [8]
♦It was too boring a book to read. [9]
With enough, the construction is more common if the adjective is premodified
by not, as in [8a]:
♦ She is not brave enough a student to attempt the course. [8a]
But with enough and too, this construction seems to be possible only if the
adjective phrase is part of the subject complement or object complement
(compare [8] and [8b]) :
♦ *Brave enough a student to attempt the course deserves to succeed. [8bl
With so, the construction is also possible if the adjective phrase is part of the
subject or object [10]:
♦ A man so difficult to please must be hard to work with.
♦ So difficult a man to please must be hard to work with.
Exceptionally, certain short prepositional phrases may also premodify an
adjective in attributive position:
♦a by no means irresponsible action
~ an action (which is) by no means irresponsible
In the following couple of ngrams for the particular case "brave man" it can be deduced that the age old dominating usage of the separation has frittered away to become nowadays approximately equal in frequency to the regular usage. From the second and third ngrams, it is clear that usage is subject to certain vagaries.
ngram "seen so brave a man,seen a man so brave"
ngram "see so brave a man,see a man so brave"
Even in this case, I believe that using "seen so brave a man" could be felt as idiomatic.
The truth of the contention about "enough" and "too" relative to the use of "not" might be shaky, as the two ngrams below show that it is not so for two cases.
The contention re the necessity of the adjective phrase being a part of the subject or object complement (and not the subject), is however, in my opinion, quite dependable ; I can't get a reference concerning that point.
There is hardly an alternative for "so bold a claim", unless one is ready to innovate, and apparently some do that.
However, for other combinations, such as "a man so tall", it is different, and both possibilities are used with a frequency that differs little from that of the other ; "so tall a man" has taken the lead again recently, but after approximately 25 years of trailing behind.
The artificiality is all relative in my opinion; the apparent aim in this change of order is first a more or less conscious intention to normalize the construction so as to have an adjective that precedes the noun (a claim so bold → a so bold claim). In doing so, the adverb separates unnaturally the article from the adjective and the noun; since it is natural to place the aticle right before the noun, doing so results in two natural blocks (so bold | a claim), and in the end the postposition of the block can be seen as equivalent to its anteposition (although, it not being usual and the order "article-adjective" being now reversed, there is a tendancy to find that artificial).
The article is still used because it is necessary to give the noun a determination, and when you want it to be indefinite, then "a" is the article. It allows, among other things, to delimit clearly a noun phrase as in "so white bread", which would often be ambiguous, and "so white a bread", where "bread" is immediately understood as countable (ref. 1, ref. 2).
