Found a (the?) solution, one can do it with the split processes (which is really not new, you could do this in the past, only had to set the stop and start print height yourself)
Setup a single process with the prime pillar. Then go to Tools > Variable Settings Wizard, and move the little slider in the bottom right until the plane is right where you want to stop the pillar from printing. Then click "Add Location" and accept. That will create 2 processes, one on the top and bottom of that location. Click on the top process (probably called ProcessName-2 or something like that) and then disable the prime pillar for that process. Now it stops when the first process is complete.
the prime pillar in 3.1 didn't work well at all with multiple processes, so this is already a big improvement!