In my limited engineering and concrete work experience, I’ve a few thoughts on the idea. For rails, steel I-beams would likely be the most stable and cost effective. And 2.5″ beams would be small enough to print attachments for on most printers. For the z gantry, I think something similar to a smaller-scale car lift type setup would be mountable with a cement nozzle
It would need a feed system similar to a gunnite/shotcrete machine. I can’t imagine a simple auger system would provide near enough flow for anything large scale unless the auger were massive.
For the concrete itself, I suspect a fiber reinforced concrete would be the most dimensionally stable way to print since verticals rebar would be impossible. I’ve used basalt fiber in parts of the ferrocement dome cabin I build on my property, and with the right additives like fly ash, it can be made to a slump consistency that could be pushable through a MPCNCrete machine.
I wouldn’t have the cash to fund the project, but I can offer a spot on my property in northwest Texas.