Entities should never touch void. And why not just make a box around where the doors are?
i use entities in void all the time. the only conflict seems to be when they start IN and OUT of the void instead of one or the other.
for example - the easter egg in tunnel_vb2:
when a dretch enters the doors, it triggers another door made of 2 brushes.
1 brush completly on the map. this is the entrance door.
1 brush completly off the map. this is the crusher that moves into the map when triggered.