Basically I think the idea can work ... if you know the programmers behind it and thus can be pretty confident that the have a good chance to manage and perhaps the quality of their work and you know the amount of money/ransom to spend in advance.
The examples mentioned, that is Martin Würthner and Peter Naulls with Gimp-Print and Firefox, do definitively fall into that category with Martin even going one step further stating that all he wanted is commitments but no money before his first release. On the other hand there are examples where the pay-in-advance for some more-or-less defined thing expects regular payments (i.e. a subscription) and delivery is much overdue thus the result quite a bit more expensive than promised since the next year subscription is to be paid.
But in any case I think some wishlist with the option to state some amount of money the user is prepared to pay would be good since perhaps that can help freeware/shareware/... programmers to find something to work on as it might help commercial companies to see where they could invest some work, or where they could mention that they are already addressing it to indeed help to avoid unnecessary duplicate developments.