I've used a Magic Eraser a lot for removing pencil marks from painted walls (carried one in the tool belt when I was installing artwork a couple of years ago), but I don't think it's abrasive enough to scratch poly. The whole reason for sanding between coats of poly is to put minute scratches in the finish and provide "tooth" for the next coat to adhere to. I doubt the Magic Eraser would do that.
That said, it might work for removing any dust nibs (bumps) left in the final coat of finish. Shouldn't hurt to try. I'd wet the Magic Eraser first, then wring it out well before rubbing on the finish.