Alright, I'm throwing in my two-cents...and this may have been covered in one of the posts including the one with the caps, or shortly thereafter. Was too much for my tired eyes to read, tonight.

Anyways, I've MANY times thought about doing something like this. But there's several problems you'll face.

1) Getting the correct angle/curve of the hatch roof is going to be difficult.
2) You're going to take away a relatively decent amount of rigidity of the vehicle in doing this, this causing greater chance of leaking.
3) With a roof glassed out that large...think of how hard your AC is gonna have to work to keep the insides cool...and same goes for your heat.
4) Everything would have to be completely custom. That's alot of work, and alot of money (granted, it WOULD be badass)
5) If you just barely cut that glass wrong...you've gotta get a whole new sheet...

The best option I came up with (thus far) is to make it more like TWO sunroofs. Put one in the normal place, and offer enough room for it to move, or whatever you want it to do, and then place one behind it over the rear seat. You would still have the rigidity (for the most part) and one could be fully functional, if not both...