Check the MEL again HB.
3 onboard, 0 required for dispatch:
(M) Associated lavatory system(s) may be
inoperative provided:
a) In the event all 3 lavatories are inoperative,
the Chief Engineer or, in his absence, the Deputy
Chief Engineer must grant permission to use this
relief.
b) Associated components are deactivated or
isolated to prevent leaks, and,
c) The pilot-in-command will determine if flight
duration is acceptable with the forward Lavatory
unusable, and
d) Associated lavatory door(s) is secured closed
and placarded “INOPERATIVE – DO NOT
ENTER”.
Do you think the engineer would grant permission? you can bet your ass on it. It will be the skippers call not to fly, unless its a 20-40 minute flight. Remember, your asking the crew to work in an environment with no relief for their duty which will be a lot longer on the aircraft than any passenger.