trackball on the cheap?

Trackballs seem very popular on homemade Mach panels etc for obvious
reasons of compactness. I was messing around with something last night
an a large ball bearing I had dropped into a counter bored hole in a
plate of steel. I was sat there rolling it around under my fingers
and it moved rather nicely, it could have been a tad bigger perhaps
but it was pretty slinky.

So here is my idea, stick a ball bearing in a hole, underneath mount a
cheap optical mouse that looks at the ball and you have a cheap,
cleanable, robust optical rollerball.

Problems:

* need to reverse mouse, I think that should be possible
* need a texture of some sort on the ball for the optical mouse to see.

Just an idea.

Cheers,

Graham