Does laundry soap count? I toss a bit in with the mix, then post-shredding about a fifth of the total weight of the mix gets added.
It reduces soap scum (supposedly, but I use vinegar as a fabric softener so never see scum no matter what I do), softens water a little bit, and removes stains.
I've never tried it in body soap, but it should also reduce hard water stains there while being very skin-friendly in the 1 Tbsp PPO range.