... The thing it's closest to is ladder bars with the engine mounted on the bars. Depending on where the engine is mounted in relation to the pivot point more or less weight will fall in the unsprung category. My feeling is it's way too much work for the dubious results that may occur...
I agree with all of that. Except that in my case the "way too much work" is offset somewhat by the fact that the nicely designed torsion bar suspension is already in place for the rear wheels. So the only "work" I'd be left with is design & fabrication of the subframe and its frame-anchored pivot. [While avoiding the "work" of mounting the rear axle rigidly]
With an A-shaped subframe, it's closest to ladder bars with their front pivots moved together. This would preserve the non-binding independent travel of each rear wheel.
As for "dubious results", I've seen a number of examples of satisfied landspeed racers using some form of suspended rear subframe.
Thanks for all the comments.