Thanks for reply.
I guess RF lenses, being made for compacts that have no mirror boxes, will have rear elements near the shutter plane... hence there is real danger of mirror hitting the lens... unless I keep it away with extension tubes... which will naturally cause infinity-focus loss.
But perhaps like the Voigtlander 125mm for Canon EOS, there may be one built for M42 or Pentax K mount. Just my guess.

Looks like they do have Voigtlander-to-M42 adaptors (
http://www.cameraquest.com/adapt_deckel.htm)