I was thinking of putting the pivot inline with the seatpost and as close to the gearbox bridge as possible.
That way the body weight is technically unsprung, but there is no vertical degree of movement available to affect the suspension, just a small fore/aft travel at the BB.
I've got several URT frames (Ibis Szazbo, Breezer Twister, Ballistic, Trek Y-frame, Klein Mantra). I like to ride them as RB tech dinosaurs, so I'm ready for the disappointment
Pivot concentric with the BB is sweet. (Like the Fisher RS-1, to keep things retro)...but difficult to fit in around the Pinion stuff.
All the best,