Awesome idea. Alternate art is a great reward for donating.
Alternate art would be great for tournament winners and donations, but I think limiting it to certain cards would be a good idea. Having a completely different set of cards would probably be a lot of work or those who make them, and some of them aren't really worth trying to give an alternate art to(i.e. Feral Bond, Mutation, etc.).
Although giving cards like dragonfly alternate art would be cool just because no one ever has those in their deck, so people would use it just to show off, therefore increasing its use. I think it would work that way, I could be wrong.