Maybe your existing wrist rest just has too much friction. I don't have any problem sliding my wrist around on the desk mat I'm using.
Your post reminded me of these:
- The Zergotech Freedom has wrists rests that slide around on teflon feet in trays. The keyboard is not out yet, but apparently they already have the wrist rests in their store. I wouldn't think they would ship orders too soon though.
- ErgoRest are separate rests for your forearms, on metal arms clamped to the front of your desk. You would have to set them up so they scissor properly.