Many polarizers have a mark (usually a white dot, or a screw) that lets you remember the position using the "x o'clock" method. Failing that, you can use the writing or like has been said, scribe a mark.
I have used a polarizer successfully with an RF (and a TLR for that matter) in the past, its just a little more of pain than on a SLR. That Mamiya adapter sounds wonderful!