a skeleton butt plate does not need to be hardened - its not subject to wear other than in your shoulder - and casing it will just warp it.
I would just dip it in salt water, urine, or suspend over vinegar in a warm atmosphere overnight until it is tarnished and then just rub it with oil.
Case hardening is done for two purposes first to harden the metal to retard wear the second is to retard further oxidation (rusting). The pretty colors are just a bonus. For my guns I prefer that they not look like they were rescued from the barnyard or feed lot.
