I'm thinking home all would move all axes at the same time. You'd probably be best to hand code moving Z first

By Default, Ref All does one axis at a time. You can modify the script to home all axis at once using the RefCombination function.

I have Romaxx HS1 and the custom Romaxx set which you can freely download has 2 buttons, 1 to zero Z and other to zero XY, you can download it from the web http://www.romaxxcnc.com/screens.html and see for yourself how the buttons are programed and then do the same or similar. I believe its a very clever solution