I too have the 645A*300/4, and queried a 645-qualified Pentax rep about options. He recommended strongly against putting a collar on the lens, because the construction of the lens tube would not sustain the weight of both lens and camera. I bought instead the Manfrotto solution mentioned above, and it works very well. If your system is Arca-Swiss based, consider instead the solution from RRS called "long lens support package". It's filed under "multi-purpose bars and rails" on their webside. With any luck this link takes you there:
Really Right Stuff - Item Listing
Notice that both solutions basically consist of a monopod head connected to an L-shaped stabiliser for the lens. If you already have a monopod head you might have half a solution. :-)