Okay ended up going with this recipe that I found in a dozen or so places on the web. Wanted to make a smaller quanity in the event it doesn't work out well, and stumbled on this...the peppermint won me over.
Homemade Peppermint Laundry Soap Powder
1 bar (finely) grated peppermint soap (try the Dr. Bronner’s castile brand or Lush, the more natural, the better)
1 cup Borax (Sodium Borate, found in the laundry aisle)
1/2 cup baking soda
1 cup washing soda (found in the laundry aisle)
I used 1 bar of Kirk's castile (what I had lurking in the cabinet), and added 10 drops of peppermint oil. My big grater/zester turned the soap to almost dust but it was a PITA to use...I think next time I'll grate it more coarsly and put it in my old food processor that is hiding around here packed away (somewhere).
Will test drive this Tues at the latest.