If you connected a treadle to a crank, it would add extra complexity and extra points of motion, or friction. Another issue with a treadle connected to the crank is that there would be points at the top and bottom of the revolution where the crank would be difficult to restart, and force into forward motion.
A pedal assisted car might overcome some of those issues with electronics.
There have been efforts to improve the ergonomics of a crank such as the http://en.wikipedia.org/wiki/Biopace [Links inactive - To make links active and clickable, login or click here to register], although I don't believe it has been widely accepted, and apparently at least the Biopace is no longer made.
One http://www.humancar.com/ [Links inactive - To make links active and clickable, login or click here to register] prototype uses a rowing action rather than a pedaling action. I don wonder if it could have had a fixed seat position, and only moved the pedals and arm portion.