Good for you, Phil. I went through GRD III & IV to the GR (and reacquainted a III recently because of its f1.9 and because it can fit in the Sea & Sea UW rig that came with a Ricoh GX100.
Beyond that little celebration of good gear in common, I never tried a hack incorporating the adapter tube that the .75 goggle screws into. But it seems that you have to start with that locking tube, and experiment from there, taping on dioptric glass from oh, a Yashica GL series or similar fixed-lens cameras. You may need an extra adapter tube to saw in half or to whatever length obtains focus. These calculations are way beyond my pay grade though. Have fun!
—I recall there was a thread about using X100 adapters, too. Somewhere in RFF GR-land.