Depends if there is any pressurized space behind the lav, and if there are any hatches in the pressure hull within that space. On most airliners I would say it's not plausible, but looking at a VC10 cutaway it looks like the pressure hull extends all the way back to the tail cone, to a small bulkhead under the rudder, and there is a hatch forward of that. IF the mechanism can be worked from the inside, the hatch could be opened when taxiing because the hull is vented to atmosphere at that point. It's possible that a panel in the aft most lav would expose a service hatch in the non-pressurized bulkhead where the engine support beam goes across, which would lead to the tail cone hatch.
Lot of ifs there, but from what I can see I wouldn't rule it out.