Buffalo horn is composed of the protein keratin so it is chemically similar to fingernails and horses hooves. I read that groomers of million dollar race horses have strengthened the horses hooves and noticed that their own fingernails became thicker and stronger when they applied hoof care products like Barielle, Gena Healthy Hoof Cream and Hard As Hoof. It might be worth experimenting on a broken old horn buttplate.
I have improved the appearance of old hard rubber or gutta percha buttplates by wiping them down with D.O.T. 3 or 4 brake fluid. I read that there is a rubber preservative in brake fluid and that antique radio restorers use it on old brittle knobs. I don't think I would try brake fluid on horn though.
