Yeah, don't want to use regular bleach at all. Not even on buck horn IMO which is less prone to cracking.
The key ingredient in the hair bleach is the peroxide. That is what whitens It, and It has been used for bleaching bone, horn, etc, for many years in several different crafts. I don't bleach Ivory at all, because I prefer not to. It's too prone to cracking as It is IMO. The ivory I use is never so discolored that It's such a huge issue worth the risk, and I don't mind a hair of character as long as It's not down right ugly. I have no idea If proper bleaching would have any ill effects, but I have bleached buck horn before with good results, although It's really strong and there is much less to worry about with It.
I don't really do Ivory collars and such, mostly just use ivory for inlay, and so I could probably get away with It, but Piece of Mind tells me not to bleach It, and so I haven't, but for all I know if done properly It may be alright. I couldn't say for sure.
Greg