A new hope: Epoxy granite and steel.

[hardenum]

I suppose it's possible gantry brace could hit if your having very different height materials but I don't think it will be a massive issue and how often are you likely to do that.? It certainly won't be an issue cutting the part you show.
Also there is nothing stopping you having both a lifting gantry for very high parts and fitting a conventional Z axis to this. This way you have the best of both worlds. Again this is commonly done on Large gantry mills, and when I say large I mean something that could mill a full size Car from a billet of steel.!!

Regards the ballscrews then it only works with one on each column. . . . How could you fit a central ballscrew and still have the material move thru the opening.

I guess some clever positioning? https://scontent-frx5-1.xx.fbcdn.net...58&oe=5FF208F9

Anyway you have me convinced(https://www.k-mm.com/wp-content/uplo...s_picture4.jpg).

Gonna take a long deserved brake before I dive into the new design. Thanks