As gorn said, that gorgeous derailer-thingy was indeed used on the fly, even in the TDF for a couple of years. I dunno if it involved pedaling backwards, but it seems plausible. They weren't FG, were they?

Edit: read it up on campyonly: this particular one was indeed a backpedaly shifter, but the later (4-speed!) version pushed the lower run of the chain, so you pedaled forwards. That was a coastie, too, though. I guess it could be made to work fixed fairly easily. It would take some seriously skilled rider to operate it.