Have you considered offering your professional services for a nominal fee? I would think $250-500 that a starting price for detailed analysis, and potential support of a mathematical proof would be a fair price. Of course for one that will require considerably more effort, that fee could be increased.
If you are loathe to take the money you could always either donate the fees, or return them to the author. The primary purpose of the fee is to filter out the random amatuer submissions that have not been well thought or checked out. I assume that you would not mind doing a few serious reviews a year, if you could avoid the spam.