Just an update, I’ve got it working for multiple walls in active view and removed the need for a custom node by updating my python.
Thanks to all who’s work I have built upon,
Mark
It struggles with angled corners as you’d need a reference ‘point’ rather than plane, any suggestions welcome 
EDIT: It’s also using the angle of the wall to work out which should be dim’d, so I’d need to get only the wall faces which are vertical and not aligned to the location line, then pick the point closest to the location line which intersected the external face… simples… Getting the ‘point’ might be the easy bit!
