I haven't seen any that lock, but if you find some please share as I would like that as well.
I've seen some that click into a mount on your saddle or seatpost. Fizik makes saddles and seat wedges that click into them for quick removal. The obvious disadvantage is you have to have one of their saddles. I know that's not what you're looking for, but might be an acceptable alternative. Here it is:
http://www.fizik.it/catalog_accessor...bid=saddle_pak