The R-D1 accepts more collapsible lens than many M bodies. My 90mm Elmar fully collapses(barely!) on the RD but not on the R3a. The many thing to look out for is interference with anything that swings out of the way when exposing like on the CL. Operate the camera with an open back at a slow shutter speed and look for problems. If you dont see a wildly swinging semaphore, give it a try.
But, I caution you, very very gently, a little at a time. If you feel anything, STOP. Touching the sides of the chamber is no big deal, but if you contact the shutter, or any other moving parts, you could seriously damage the camera.
This isnt really rocket science but enough people have screwed up their body to warn about being obvious to the dangers.
Rex