Silly question perhaps, but have you tried using the MakerVerse menu option shown below once you’re at the z-axis position you want to define as your machine zero? This would also suggest that “G28.3 Z0” should work - but I haven’t ever tried that myself.
I’ve used the MakerVerse menu option successfully with v1.1.2 for both the x- and z-axes. It did not work for the y-axis (see Can't set machine to zero - #4 by bilzer046 for more)
