How about a water-pick, tuned to a degree just hard enough to tickle like crazy without going in to the pain area or maybe just slightly into the pain area so, that it's all the more maddening?
Or a rotating wheel at the end of a table with a brand new tire that has those little rubber things sticking up like tiny fingers. That would be absolute torment for anyone with sensitive soles, strapped belly-down, soles up.
An old carpet shampooer with dual-rotary plastic, bristles works great too! I've tried it out and it's next to impossible to keep your feet still with those little scrubbers whirling on your soles!!