There are numerous possibilities to add display enhancements, like the one you mention, but also in (future) cases where we simulate switch failures of some kind where you need to send a repair crew.
A dot might be difficult, though, since we use dots already for slip switches to indicate which side you can move from one crossing track to another. Besides, on those switches you have two motors which can be locked independently. A grid element can be awfully crowded there.
Possibilities are using different colors and also to display the lock status in the status line when the mouse hovers over the object.
