I drilled a Mamiya body cap 1/2 in. and used a very thin piece of tooling copper with a 0.40mm hole cemented behind it. I use the shortest extension tube, which gives a more or less "normal" view and covers the 6X7 format just fine. I haven't tried the cap directly on the body for a more wide angle look. Should make a smaller hole for that I guess.
That's the better method, large hole in the cap and a metal insert/section for the actual pinhole.