I personally recommend the use of mineral oil for seasoning the wood items and here are my reasons: #1 Mineral oil will not support bacteria growth as it is mineral thus preventing the growth of bacteria in the many cuts and crevices which wood will acquire after a while from use. #2 Granted vegetable oil may be period-correct but the mineral oil is safer and gives the same effect.
In regard to frizzens (I know what you're going to say but unfortunately most of the Alliance knows them by this term). I have built around 33 guns -- two were percussion, five were from kits and the rest were from scratch (including the locks). In all of my 20 years in the NWTA I have hardened probably 22 frizzens (or hammers as you [correctly] refer to them). The Japanese Bess's are the worst as they seem to be made of malleable iron, while the Italian Bess's and all of the Charleville's are cast steel and last longer. Another point is that we fire them a lot more than they did in the Colonies for no one has any records showing replacement of gun parts and of which ones they did replace.
Even a gun with a weak spring will fire if your flint is sharp and doesn't have a hard spot in it. A new spring will allow the flint to cut through even the thinnest case hardening until it is gone. I can check most of the guns in camp and have even hardened hammers in camp for a modest price of two candles (which pays for the gas torch which I use). About one out of three guns that I check have hammers in need of hardening -- most misfires are the result of loose flints or hard spots.
Period correct is one thing, but proper and safe methods -- whether modern or otherwise -- should always take precedent over all.