I had considered this in the past. I thought it would be neat if Stack Overflow had something like Google's summer of code - where we encourage our users to dive into open source projects and bring them much needed features or bug fixes.
It then occurred to me that our users already do that and we don't have the capacity to manage a program like GSOC, and should not kid ourselves that we do.
So, this year, I plan to pay much closer attention to what our users are doing and reward those that have gone out of their way to make something that people care about better. If I can pull it off, folks will just be pleasantly surprised upon receiving an email from me asking where to send their shirt. I hinted at this when I talked about getting users that aren't likely to ever hit the top 50 or 100 user lists some of the stuff that we distribute.
I fully agree with Shog, we don't need to codify more extrinsic motivation, it almost never has the desired effect. However, spreading the wealth that we do distribute a bit more evenly has been on my radar for some time. I'm definitely going to make this part of my job, it's something I feel we should do, it's just a question of when. Heck, I'm backed up in fulfilling some of the prizes from Winter Bash (I promise, they're all going out this week, I got sucked into some unexpected abuse related issues every time I sat down to do it).
The crux of this will probably be the community moderators on our sites. I'll ask them to keep their eyes open for those individuals that aren't ever going to be reputation giants, but remain passionately and openly committed to the same philosophies that our sites are built upon, and I'll make sure to brighten one of their days.
I'd much rather thank someone for being consistently awesome than hand someone a prize for being the first to solve a puzzle. That's, in my opinion, a much better use of our resources, and much more likely to be valued much higher than the actual cost by the recipient.