Yes, I take your point; the thing must be "idiot proof". We have had some problems already with people (mostly male I think - must ask the secretary about this ...) simply not locking the door, mostly more from general vagueness (habit?) than not understanding the switch.
The significant complications are only in regard to the ability to unlock the door in "emergency" situations - specifically, someone collapses inside. That has happened in the past, hopefully not too frequently, but always a significant risk.
Other than that, it is simply a switch marked "Door lock", presently with a red/green LED. Only the fact that it is not actually on the door itself differentiates it from a conventional snib. A display which says "LOCKED" would be a logical extension both on the inside and "OCCUPIED" on the outside. There is already a red LED on the electromagnet at the top of the door frame.