First look at installing light fixtures into a ceiling (versus a wall):

In the draw menu you change the "Location" from Wall to Ceiling and then the plugin allows one to insert light fixtures into Medeek roof and floor assemblies or into any group.
The control key toggles justification: LEFT, RIGHT, FRONT, BACK, CENTER
When location is set to "Wall", the justification is only: LEFT, RIGHT, CENTER
Light fixtures in ceiling mode can be inserted into any group/face that is not vertical (normal vector of the face is perpendicular to the Z axis), so any horizontal or vaulted ceilings are okay.