Are you sure you can't buy a I made to size for your rifle? That would be the best bet, second to that a for probably won't touch the rubber. However some 80,120 or 240grit sandpaper would probably do it. If you stapled a strip to a piece of timber and rubbed the recoil pad on it I'm sure you'd get it done.
I'll probably get in trouble for quoting a mate but...a mate told me if you put it in the freezes the rubber will harden slightly making sanding easier.
Be sure to tape up the stock and mark the pad out accurately. Good luck with it.
Chronos