Just saw that this page of the PDK mentions a p-well resistor alongside the n-well resistor. I couldn't find the schematic symbol or model by grepping inside the tech, so I'm curious if there actually is one.
-- Not actually planning on using them, I'm just curious.(edited)
Unfortunately, I don't think it is part of the PDK.
The resistors page does not list it: https://gf180mcu-pdk.readthedocs.io/en/latest/analog/model_parameters/LV/LV_1_4_4.html
Looking at the spice models in the PDK (sm141064.spice), I can see that they only include the ones shown on the resistors page.
Magic, however, does extract resistance for pwell/isosub during pex.