TL;DR: The uses of will and would are very complex, and it's not always possible to make a clear determination of the function. I think that's certainly true of would in your example, and as a result, I suggest it's safer to stay with the accepted idiom "if you will", even if "if you would" might be technically correct in grammar.
Let's look at the two words in more detail in the context of your sample sentence.
He wasn't very bright; kind of a dummy, if you will.
The expression "if you will" is somewhat idiomatic. Oxford Living Dictionaries defines it as follows:
if you will
Said when politely asking someone to do or consider something.
- ‘imagine, if you will, a typical silversmith's shop’
In the context of your "dummy", "if you will" is another way of saying "if you wish [that I put it that way]". It's equivalent to "if you like".
A more comprehensive discussion of the phrase can be found in this EL&U answer, where RegDwigнt confirms your thinking that will is being used here as a full verb meaning want or wish, rather than in its more familiar use as an auxiliary.
He wasn't very bright; kind of a dummy, if you would.
On one level this is no less correct than using will, as would is simply the past-tense form of the verb; in that sense, "if you would" is another way of saying "if you wished [that I put it that way]".
However, this is where it gets complicated: would in the past-tense sense (wished) is an archaic usage, while paradoxically there are some contemporary – if limited – usages in a present-tense sense (wish). Merriam-Webster explains:
12a. strongly desire : WISH
- I would I were young again
—often used without a subject and with that in a past or conditional construction
- would that I had heeded your advice
12b. archaic : WISHED, DESIRED
12c. archaic : wish for : WANT
Another interpretation is that would is functioning here as an auxiliary to introduce an irrealis mood, albeit in an elliptical construction where the missing verb (e.g. "prefer", or "allow me") is understood: "kind of a dummy, if you would [prefer]". To quote another EL&U answer from RegDwigнt:
Would is also used as a modal verb to indicate a conditional or subjunctive mood, or to "soften" what is being said:
- I would love to see that movie.
- If I had a hammer, I would use it as often as possible.
- Would you give me that book, please?
There's a further complication: the idiom "if you would" is typically used to provide a polite gloss to a request, instruction or demand, in which case it is equivalent to "if you wouldn't mind". The Free Dictionary defines it thus:
if you would(, please)
If you are willing (to do something).
- Fetch my coat, if you would, please.
This usage doesn't quite fit the context of your sentence. Since "if you will" is the more common idiom to say "if I might be allowed to use such a description", using "if I would" is more likely to be mistaken as a marker for a forthcoming request such as "pass me the bourbon". When the request doesn't materialise, the sentence seems to be left hanging.