Ok, I think I may have an idea for a taillamp, after checking out the cool site link SeaKing sent me. As usual, I had to dream up something that will require more work than I care to invest, but what the hell, I've got nothing but time, right?!
My thought is to take a piece of, say, 2" OD tubing, perhaps 3-4 inches long, do a mitered cut along one end to fit the contour of the inside of the tailpiece. Then I'll cut a 1.5" hole in the back side of the tailpiece and weld the tube in behind it. I'll need to weld a backing plate over the open end of the tube inside the tailpiece, with a hole for mounting the light socket. I can also cut a 2" disc out of the existing taillamp from the GS650 as a lens. Around the circumference of the hole I'll drill several small holes for either rivets or small screws. These will secure the lens tight against the inside edge of the hole. My paintbrush illustration below kinda shows what I'm thinking: