Sorry if it has already be proposed before, but for a Z-axis calibration, couldn't we just print a predetermined shape, for example, a tube (which is the easiest with two sinus signals) for a know quantity of drops and then, measure the final print with a caliper (yes, maybe not everybody have a caliper, but at least, the poeple in the beta team).
So, for X drops, we've a tube of Y mm and then, we know the thickness of the layer added by a drop. These could be parameters for the slicer, or even be included in the Peachy Printer software.

And not knowing the Z-axis speed, I suppose there is no problem to print the tube at "full speed" for the X and Y axis, several passes on the same layer shouldn't cause any problem... I suppose.