I don't see how a pipe connected to a cylinder connected to a header tank
which is open at the top CAN go 'overpressure'

Scale. User error (sticking a cap on the end to stop air entering the
system). Blockage by organic matter (i.e. a nest of some sort).

Christian.

I would thing that the 2 bar pressure necessary to get the water to 120C
and in contact with someones skin without turning to vapour would be
pretty debilitating if maintained for any length of time.

(a) not at all.
(b) not required. It doesn't have to stay water for very long, so the air
pressure doesn't have to be elevated.

Christian.