I think that's a question that has plagued most of us. The remanufacturing of the westinghouse laundromat door boot comes to mind. I don't know if you can restore it... Then again, maybe someone knows something I don't.
I've used a product from Caig Labs called CaiKleen RBR for cleaning rubber idler and pinch rollers in HiFi tape decks and turntables. Caig calls it a "cleaner and conditioner" for rubber and it did a great job removing the glazed surface and softening the roller. I doubt it will do much softening past just the surface so don't expect miracles. Also its a bit pricey but for tape deck pinch rollers it works great!