I don't remember who it was, but one PCF member successfully sanded some hotstamps. May have even been River City chips.
IIRC, he took his kid's Sponge Bob battery toy that has a rotating shaft and somehow hacked it - it was like a dremel tool, but slower, and with a small circular piece of sandpaper that was flat at the end, and smaller than the inlay, so it didn't touch the clay part of the chip.