To be able to dimension openings in walls you have to do some filtering.
First you need to take the vertical faces of the wall which you can do with Surface.FilterByOrientation from Clockworks package:
Then you need the faces that are perpendicular to the dimension line. You can filter them like that(this is the principle for the filtering inside Surface.FilterByOrientation too):
Then you need the references of the actual wall faces. I did this with this little node I found inside Wall.ByFace (Clockwork again):
And finally create the dimension with a simple Python node feeding the line and the references to it:
And you’re done.
This of course works only in plan views. The method could be refined to work in sections and I think all of this could be done in Python for better performance.