I've read about people feeding their horses eggs before. I thought it was fascinating! I don't know if it's bad for them and I've never tried it myself. If you feed it consistantly and your horses are fine I don't see a reason to worry. There are only a few organic supplements I know of that can have great short term effects and bad long term side effects, like uncooked garlic which can make horses dangerously anemic after a few months (or something along those lines).
There was a post on here a long time ago about someone's horse stealing eggs from the egg basket in the arena. His owner was worried that it might make him sick but nothing bad came out of it. It was pretty funny, actually, I laughed out loud when I saw your post and thought of the owner's story.
Sorry that I'm not much help :P
I am here to learn! :)