Well, don't know why you aren't getting electrical pickup from the rear tabs. I'd probably look into that first. But my solution remains the same. Get an 00-90 round-head screw (or really, any kind of head would do for this), tap drill, clearance drill, and tap. You can get the tap and drills from Micro Trains, since they use 00-90 screws for couple body-mounting; I don't think they have a 1/8"-long screw, but you can get 1/4" or 3/16" round-head 00-90's from a variety of places (you'll have to order a package of them, which usually means a dozen or so).
Pick a spot on the frame where the screw, thin piece of phosphor bronze and wire won't interfere with the shell, drill a tap hole, tap, screw, solder, done. I might put the screw on the thick tabs sticking up in the center section of the frame, as long as that won't interfere with the shell fitting down on the mechanism; if that's not good, use the rear shelf and a longer wire.
John C.