I'm assuming you use a standard seatpost. I personally can't think of quick release system that would be sturdy enough, but i think it would be easier to come up with a way to attach it to the saddle itself ( maybe using handlebar extensions of some kind). As you would have guessed if you mount it to the standard seatpost, it won't retract far enough to lock the bike when folded. you could also try to attach something to the bottom of the seatpost (http://www.sjscycles.co.uk/brompton-...nga-prod13548/) to slightly increase it's length) or the final option would be to by either a telescoping or extended seatpost.