difference between rack mounts and fender mounts?
There's no difference that I'm aware of.

My bike has 2 sets of eyelets, and I have a fender screwed into one set, but I could just as easily use the other set. I suppose the same could be said of using either set for rack mounts.

Looking at it from a structural point of view though, it would probably depend on the bike and the materials it's made of. But if it's in the right place, I'd probably give it a go :)