The IR LED solution would not work with white acrylic or perspex I don't think. It would need a transparent tabletop.
I don't want to run wire across as this would be visible, no matter how fine it was and it wouldn't be possible to affix it to the tabletop without it being clearly visible or without it creating a bumb on the surface or attracting dirt. I want a nice, clean smooth white surface.
Any other ideas? This must have been done before I would guess.