I was thinking about this for my machine but I dismissed it because I though it would put too much axial force on the Z-axis rails. Some of that could be addressed by mounting the z rails facing forward on the front of the riser columns but.... ultimately, the goal was to increase rigidity and I concluded that this was better achieved with a fixed gantry.

Still, a z-axis protruding below the gantry can't be great for stiffness. I am still wondering if I should have made the table move up and down for the Z-axis instead.