So according to the wiki, if you poke(0x5f2d, 0x4), it turns on pointer lock mode, and then you can use stat(38) and stat(39) to read X and Y mouse movement. This isn't working for me, though. Both stat()s just return 0 no matter what. I'm not sure if you have to poke(0x5f2d, 0x1) first too, but either way it's not changing anything.
Here's a little demo cart to demonstrate:
I haven't seen anybody else use it, so is it even possible? The only thing saying it's possible is the wiki.