But if you even notice the (thin, dark?) bit of cord between a guy's wrist and laptop computer when he's working on it at a table in the library, wouldn't your first assumption be that he's a bit paranoid about having it stolen and therefore has strapped it to himself by the Kensington Lock slot? So you'd be quite OK with stretching it as far as it goes -- only noticing later that in this case you could "stretch" it beyond that, popping out the USB connector. (Actually, wouldn't a lot of people in a position to do this be only too happy to give it an extra forceful yank, just to hurt the Eevul Hacker?)
Maybe there needs to be an accompanying/alternative device which can be worn in a shoe and detects toe movements. It would probably have to be wireless, which would introduce false positives or false negatives, (and part of it may need to be attached to the user's ankle, due to size constraints), but it would at least defend against an attacker who could physically restrain the user.
