Have you found a commercial machine that uses your structure / axis setup? If you can't, that should tell you something.

I've never seen one (and I've spent a lot of time looking at machine frames / designs).

There are large machines with a Y axis at the bottom, a lifting Z beam, and X axis travelling across the Z beam

Axis naming convention for a vertical machining centre:
Standing facing machine
Z goes up and down
X goes left and right
Y goes forward and back