Have an old, hard red Pachmayr ventilated pad on a sportered military rifle. Once these get old and stiff, are they goners or is there anything that can restore some life and flexibility to them?

Nope, throw away.

Boil it in linseed oil...maybe there will be a viable use for BLO on gunstocks after all.

Fire starter

If just the outer skin is dry and cracked you could sand it back to fresh rubber and fit to a smaller butt stock. If dry and collapsed sand at a 30 degree angle and use as a door stop.

Toss it..