I found a workaround (I think).
If you set up the labeling parameters in a 2D window, then copy/paste that layer from the 2D window to the 3D window, then those layer properties carry over. So - I keep both windows (2D and 3D) open, and "Link" them so that they are synchronized views (optional). I then use the 2D window to tweak any properties that need tweaking (including symbology, etc.), since it renders faster anyway. When ready, I then remove any old version of the layer in the 3D window and replace it by pasting the revised 2D layer. Doesn't seem that all labeling parameters are quite right though ...
I also do some data editing, like building height values used for extrusion, in the 2D window, and the changes reflect in the 3D.