Elsewhere on this forum, there is a current thread where someone posted about having a custom adapter made up so he could use a Leica diopter adjustment lens on a Contax 11a rangefinder. Did not cost that much.
http://www.rangefinderforum.com/forums/showthread.php?threadid=101965
Seems that it may be possible depending on the diameter of the exit pupil on the RD-1. You may wish to investigate doing something similar with a suitable magnifier if you can find one of appropriate diameter - would one for Leica R work?. ( I imagine an add-on magnifing lens could be problematic if it were too small in diameter for the camera finder's diameter.) I am not sure whether the distance from the finder caused by having an adapter would be a problem for a magnifier. If it is too far back I suppose the exit pupil effectively becomes smaller.
Incidentally, I see what you mean about the RD-1 magnifier from Megaperls / Japan Exposures. Their site says the are no longer being made and they are out of stock.
One final thought. I think I have seen some generic finder magnifiers advertised on eBay to suit Nikon and other SLRs - some of which might suit.