Matt, all 3 lenses are K/L lenses, purchased new by me. I tested the lenses with and without the mirror lock-up, and could see the shutter opening each time. However, you might be right about the cable release. I'll re-do the test with the other cable release.
If the shutter doesn't fire, then (AFAIK), the camera cannot be recocked; at least that is the case with the RZ. So if the camera seems to be working, the lens is probably firing. Whether it's firing with the right shutter speed is another matter...