Hacking Optical Mice as position/distance encoders

Hey now, I confess my ignorance on the general topic of encoders. So,
this might be a rather naive question...

If optical mice were sold as optical encoders, then they'd cost a mint!

Sooo, has anyone thought about hacking optical mice for use as CNC
optical encoders???

Issues that have (randomly) come to mind:

1.) they ain't cheep. Hopefully, they'll start appearing as used/
surplus components

2) how do you hack that USB???

3.) what are the alternatives to this approach?

4.) what resolution is reasonably accessible?

Finally, if anybody has some good references on how these things are
engineered/designed, then I'd appreciate a posting. Keywords or likely
sources of such info will work too...

Thanks! Mike Pinkerton (San Rafael, CA)