Basically it's just more sliding joints to cause a problem.

If the gantry doesn't need to move during a job then it doesn't need precision rails, it just needs to be adjustable with clamps to secure it in place.

You could have an extended Z axis (requiring long strong precision slides) or make the whole gantry as the Z axis. The mass will be similar to a Y gantry,so will require a similar motor/drive. It would require spring/air/mass to balance gravity and you'd have to find a way to stabilise the Y axis (i.e. dual lead-screws). Complex maybe, but do-able.