is there a specific way that bike builders build mounts/eyelets for racks for panniers and eyelets that hold fenders? I'm specifically thinking about eyelets on the rear dropouts. I know that if a manufacturer doesn't want a rack at all he or she won't put mounts on the seatstays, but is there any structural difference between the two?
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 :)
No, there is no structural difference. Bikes with two eyelets are meant for a rack and panniers, but are basically the same tabs.
If you only have one eyelet, you can attach both rack and panniers to the same one, and use p clamps to secure the rack to the seatstays. A lot of racks come with hardware to do this.