I’m sorry, but are you expecting even more of Mitch than he already has given? Personally I think he is well worth a donation without asking anything more from him.
Domain name costs money, webhosting costs money. Especially if you have a lot of traffic and need reliable service (trust me, the cheap web hosts may offer " unlimited bandwith" but that is rarely the case in reality).
If he gets more funds over what the pure running costs is, then kudos to him. He can maybe take some time off work and adress fixes and improvements. (You get more done on a 10h day than a 2h day). But this is (for me at least) not a demand. As long as the donations first and foremost goes to keeping this forum and the repository online, any left overs Mitch can do whatever he wants with.
And, “one time” donations is the better choice in my opinion. Just don’t do the awful nagging that wikipedia did/does. (I donate, but I wan’t to do it on my terms).
And a title that says “sponsor” is the best option (with a 1 year time limit) as a “reward”.