Ok, well I tried a geometry nodes setup from here using vertex painting instead and that didn’t seem to work either, but maybe I did something wrong.
Another workaround I guess it to sculpt it the way I want, and then try to figure out how to place a camera accurately in order to render out a texture using mist… but can you get a sculpt brush to only make changes in the the Z direction?
EDIT: Sculpt plane Z plane apparently… so working backwards it is!