I just tried this section of the game again (from an earlier save and from the auto-save created as you get near the door), and as long as I did not enter the summoning chamber far enough to trigger the cutscene conversation with the undead Sassan, I could leave, take the elevator down to the morgue shrine and teleport back to Broken Valley. If you enter far enough to trigger the conversation (3 or 4 steps past the door) the door will be closed and locked, with no way back.
