After thinking about this, I grabbed a small bar of baby soap that is over a year old. It's 95% OO, 5% castor oil, no fragrance or additives.
It was ok, and my teeth felt really clean after using it but my husband didn't love it. He felt he needed something minty to feel his "mouth wake up". He did use the bar in the shower after that, and said he loved it for his hair and body! Go figure.
So I went ahead and did a 90% OO/ 10% Cocoa butter soap, 3% superfat (just to avoid accidentally being lye heavy) and added peppermint EO "to smell". After saponification, it doesn't smell much of mint at all, though. It's fairly firm today, and I might try a smidgen even though it's only two days old, just to see what I think.
I think I"ll do a baking soda brushing every third day or so, and keep using my waterpik twice per day.