Just to let you know that I have exactly the same problem with my Clio (98, S reg). Originally the off side rear passenger door would just swing open as the lock mechanism would not close (which is not good when you're driving merrily along). Manually locking the door by pushing down the lock button did the job, but a little too well as now the lock will not open at all! So off with the door trim and after many hours of pulling, pushing, prodding and swearing: nothing. So off I went to my local Renault dealer for advice, to be told that the only way to open the door would be to remove the external door handle and smash the lock mechanism with a hammer!!!!
In all I agree with cliodiesel that this is a matter of concern to know that our cars are susceptable to deadlocking with no means of escape for those in the back seats. It would be interesting to see if other Clio owners have experienced the same problem.