What is the feature you want to see?
Global option named extra_gallery (bool), reset on reload.
When set, spawns an additional door on the southern gallery wall. Interacting with it will open a menu with options. "Which add-on gallery do you want to enter?" (thinking animation)
Upon selecting which, the player opens the door and warps to the corresponding gallery room. Trying to open the door from the other side just warps you back, no choice. Spectators get the choise upon flying into the south wall, and warp to the dimension directly.
This room is located in a new dimension (just for this room), and opening the map will show this room only when opened in it. The door is displayed on both sides.

Why would you like this feature?
Add-ons adding more portrait ghosts will end up clashing with positions otherwise.

Any alternatives you've considered?
Adding paintings to existing rooms. Prone to clashes.
Adding the room to the overworld. Still has potential of location clashing, and breaks GBH map.

Additional context
It may be a good idea to change the SelectedFrame score to check the dimension, and not just the value.
Potential build-in gallery designs matching the beta galleries.