Thank you Steve M. That helps a lot. Knowing that it's possible makes it worthwhile to try to track down an example.
Thank you, too, StevenK. I have seen some examples by the Chomin that Davey lists with that signature, and they all seem to be ivory. It was suggested to me that I check Dieuwke Eijer's book Kagamibuta, since there is a Chomin mentioned there that might have been a metal worker. The person who made the suggestion posted the signature that Steve M. posted above. I don't know whether that's actually in the book though. If it is, then the problem may be solved. At least it is helpful to know that the signature on the netsuke could possibly be read as Chomin, since that seems to be the most promising line to follow for now. I've attached a photo of the netsuke.
I the Davey book the closest kakihan that I could find is associated with Ono Ryomin. The signature of Ono Ryomin IV looks close to the signature on this netsuke, too (except that the Ono part is missing). All four of the Ono Ryomin's worked in ivory though, and I think that I need to find a metal worker.