Could the cabin crew open the lavatory door from outside when someone locked the door from the inside? On a recent flight a passenger locked himself in the lavatory for almost the entire duration of the flight. It was only when the flight was in the final leg of descent that he could be persuaded to come out.
In similar situations, can the cabin crew open the door from the outside if it is locked from the inside?
Apparently yes. It has been mentioned in a number of places. One of the methods given is pretty simple:

Step 1: Approach locked lavatory
Step 2: Lift &quot;LAVATORY&quot; sign
Step 3: Slide the knob into the unlocked position

Thats all to it, though they may vary with the aircraft.

As @RalphJ points out in the comments, it should go without saying that unless you are cabin crew, you should not be doing this.