One possibility (and my guess) is that the lens was designed for Ricon, and has the "Ricoh pin" which is the common cause of a stuck lens. Here's
one old discussion of the problem on DPR. Many have recommended feeler gauges to retract the pin out of the way, but the technique which has worked for me (the one time by accident, and others for experimental purposes - not stuck on a camera, but on a cheap TC) was to keep applying a torque in the direction to remove the lens while rocking / wiggling the lens... the constant torque will hold the pin against the side of the hole it's stuck in while the rocking works to compress the pin against it's spring and eventually it's forced back into the lens mount and the lens will turn again. I've also found brute force to work, but that was when not using a camera (just the teleconverter to simulate the camera) and I'd be leery applying a hard twist to a nice camera. Googling for "stuck ricoh pin" ought to find a lot of references.
If in doubt a camera shop ought to be able to help you out... I know the feeling you're going through - don't worry, your camera will be free again soon.