Had always thought about that as well and especially to tie in nicely to an event. My naggings were as follows: what if it were a poorly authored custom and someone pledged quite a bit of money, would that be fair for either party? could you "choose" your author or at least have a say in it? Like, if an author signs up for one you want and you have the option as the requestor to reject or accept that author. I realize that could lead to other issues.
as an alternative or even in addition, what if there were auctions that were like "from" authors that an author could volunteer their services for "any song of someone's choosing" that fit their style more. so an auction for charity and the result is a custom that the requestor specifically will know the result. or a group of authors who are known well for their specific styles all author one or some together?
whatever the outcome, i think it's a great idea and if vetted out correctly, could result in some fun songs and more money towards a great cause.
I don't think people making pledges should get to choose authors because it's very likely that there will be multiple people offering to donate for the same song (which is actually the point...to drive up donations or Make-A-Wish), and they may have different preferences when it comes to author. I'm presuming that the authors can figure out a way to decide amongst themselves how to dole out the "high request" songs for this event. As authors, you guys should obviously be able to pick and choose songs that *you* want to do, too...not to be forced to do something that doesn't excite you.
Your other idea would also work, I suppose...it's really up to you guys if you want to offer "collaborations-for-bid" or whatever. My only recommendation would be to keep it general; presume that there will be *multiple* people who want the same song, and instead of making it a bidding war situation, we want to try to capture *all* of the donation offers for charity, not just the winning ones!