Brian,
now I have had a shot at drawing a JR mainsail for that cat. The resulting 8-panel sail turned out to be more than big enough, but the mast also gets very high. I would rather suggest using only seven panels, which is big enough and brings the mast height down to 12m. Remember, on a JR there is no way one can fit lower shrouds. Still, with the wide possible staying angle on a cat, and a LAP of just over 10m, it should be possible without ending up with a too heavy mast section. I am not sure of the engineering necessary to handle poles under compression, so I hope others will step in here.
My thinking is that this hybrid rig, with a cambered panel junk sail as the main, will by far be the easiest and least risky rig to make. You still get the very easy reefing of the JR, almost the same SA as the BR, and that with 2.5m shorter mast. With the modest jib on a roll, you will have a handy rig.
Arne