David, did you ask about this at tonight's meeting? If not, perhaps we can Zoom tomorrow if you haven't gotten an answer, I do not really understand your question.
The whole design of the Nexus, Fe-pi and RPi is ment to function in a stack configuration. You could also use a 40pin cable, to connect a RPI to the Fe-pi Nexus board I believe.
I've got a medical appointment tomorrow so will be out from about 8:30-9:30 or so. I should be home otherwise. I can be available anytime from 0630 on save for the appointment.