Slightly off topic, but was told this by a mate who went Flight Engineering.
He was on a 707, full economy migrant charter, the Technical Crew could not get to the toilets for a break, so they put on the seat belts sign for a while and managed to get their break.
If I remember correctly on the DC-8, the Flight Engineer would open the flight station door and when someone exited a forward toilet he would press a button that locked the toilet door. The Tech Crew could then get to the toilet and unlock the door to get their break.