There's this thing:
http://cgi.ebay.co.uk/Accessory-Digi...ht_2062wt_1165
It's not for twiddling with your hand, but it definitely fits the description of "like a TIP 3rd hand but cheaper and smaller". There are actual metal rods that can be mechanically linked to a pot shaft, but it's probably more than you'd want to spend.
Apart from that, of course, most pedals can be modded to have expression control over most features, but the mod plus an expression pedal is again probably more than you want to spend.