I see no issue with it, and Leslie raised some good points there.
If you're trying to give back, are your sure that hosting/web space is what your mods would really be after? I don't know what your forum is about, but it may be that something else would come more up their alley. Maybe give out a book each month/quarter/whatever, or some kind of tool? Pick the winner's name out of a hat.
I have some T-shirts from a forum I moderate, and I won a Lexar Jumpdrive on this very forum a few years ago, that I still use for transferring small files. See, each time I use it, is an opportunity to remind myself of this place.