I too would use a RotoZip, or Dremel, if it didnt move so much, whats the point in cutting it, if your going to have to shape it after your done cutting, you should be close enough to start sanding when your done cutting, not having to spend 30 minutes with a belt sander to even the board up first.
Now, if you have some kind of Jig, that prevents the zip from moving in an undesireable path, then go for it!
Just be warned, freehand Rotozip or dremel or any other rotary tool (even routers if you dont hold them firmly) is a guaranteed wavy line.