ya that is very very neat! our school has a display that plugs into our TI-8# calculators and sits ontop of an
overhead projector like that to show what the teacher is doing. I also managed to convince one school to donate a comptuer
projector that worked in the exact same manner (but in color) when the other school was getting real projectors. This device was actually made to do this, however it was very old and had VERY low resolution. It was hardly worth the effort. Now our school has computer projectors in every room so using that old thing is rather pointless.
The one thing i would be conserned about with a setup like the one discribed on that
page would be distortoin. I dont think i ever remember any overhead projectors I have ever worked with actualy projecting a square
image. It is probably not a huge deal and maybe could be ajusted out without a huge deal of work.