If you get into the IC2 crop breeding, there are some plants that produce leather, I believe, not sure if they're in this particular modpack, but worth checking out anyway.
Some modpacks I've seen enable passive mob drops which is handled by one of the Core mods. Jason, is there any chance you could take a look at enabling that and customizing the rarity of the drops?
Also a possibility would be to fill out the IC2 scrapbox recycling loot table and add in random drops of passive creatures, some of which I believe are already in there. I don't see a problem with them if they're done correctly, and at relatively low rates for the chance of getting them.
Cheers ...
BrickVoid