As you suggested, one factor is whether or not the felt is glued all the way, or just at the front edge. If it is glued all the way, you might be able to cut the glue bond, but then you'd have hard glue in the felt, so you probably wouldn't get noise reduction that way.
The quietest design is two or three layers of felt and cloth, glued only along one edge. That's what some of the high end German manufacturers do. You might be able to devise a sandwich that duplicates the existing thickness, so you wouldn't end up with a regulation disaster. Alternately, you could also duplicate the existing cloth's thickness (just replace with one thickness), but Glue the replacement only along the front edge. Jurgen at Pianoforte Supply has 5, 6, 7, and 8 mm thicknesses, one of which should be very close.
I doubt needling or Profelt would help. I haven't tried it on back rail cloths, but I have tried on key end felts to reduce the impact feel, and couldn't tell the difference. Similar lack of results on wipp cushions and the like - lots of work, minimal permanent improvement. To experiment with little time investment (besides R & R the action), you could steam the cloth with a travel iron, then follow with a heat gun to dry.
It is possible some of the noise comes from the knuckles and perhaps wipp cushions. There is some bounce that occurs, so often return thunk comes from a few contributing factors at once.
Regards,
Fred Sturm
University of New Mexico