Thanks for the input guys - the old hub idea sounds promising, and if I ever ended up doing something like this on a regular basis then I'd give it serious consideration. Bungees worry me a little too, I'll make sure I don't have my head too near the large one when I'm hooking it on.

tsl, nice to see someone else has already tested it! Perhaps with the forks on the back it adversely affects handling by acting as a pivot, whereas on the middle crossbar it's above the rear hub so wouldn't have a turning effect. Or maybe the extended wheelbase from having the trailing wheel further back would change the way it tracks around corners... I'll have to think about that one! For now I think I'll leave it where it is and use a velcro ankle-band to hold the crank in place (so long as I don't try to push the bike backwards, although that'd be an interesting test of the shear strength of velcro).