[Papervision3D] 3D X and Z from screen X and Y if 3D Y is always constant?

[Nick Kuh]

I have not doubt that this is possible by extending/using some of the
existing classes and methods in PV3D but I need some guidance.

So, I know in my scenario that the 3D co-ordinate I'm looking for always has
a y value of 0. What I want to be able to do however is retrieve the x and
the z values to from 2D x and y  co-ordinates of a point in a rendered
scene.

Anybody?