If I understand your question correctly, you could try using the 3M/Scotch-Brite bristle disks - impregnated with abrasive media. Depending on the stiffness chosen, it could be a good option for prepping the undercoating without doing much damage to it. Minimal pressure, slow speed, and don't linger in one spot - it'll melt and smear the PVC undercoating. You may not get it all but, it should abrade the remaining paint sufficiently. And as Jonny said, it likely wont be very thick.
I used the radial version of the bristle disk for removing a flaking repaint from the undercoating on the underside of a short hood. It worked rather well without affecting the texture.
Alternately, could try Scotch-Brite pads on a DA sander (or similar) if you're happy just abrading the surface well enough for paint adhesion.
The only reason I see to try removing all traces of paint is that you may see the odd blue stone chip down the road.
Before you start, do your darnedest to remove all traces of dirt, grease, etc so it doesn't get ground-in and cause problems. I've used Castrol Super Clean and a stiff plastic bristle scrub brush, power washer, etc. with good results. Followed up with wax and grease remover.
I believe the undercoating had less texture as the years went on but, if you need to repair some spots, here is some good info from DarrylID's site ('66 912):
DarrylD's Porsche 912 Project Page - Phase 9: Front & Rear Floor Panel Replacement
Let us know what you end up doing.
Tim K