Thank you very much for your ideas and proposals! The final solution was proposed by Shalmeneser in the FreeCAD forum.

The #FreeCAD forum is just amazing!

First reply after 40 minutes - reply with the solution I'm going to adapt after 59 minutes. Shalmeneser nailed it using a DatumLine and a DatumPlane. If you're interested, I've asked some nerdy detail questions in the forum 🤓

My example showing the proposed solution from Shalmeneser is attached.

Cc @NUeB @rrmutt @chrishuck