AFAIK petrol can only damage a diesel engine once it gets as far as the
high pressure pump. As the high pressure pump is driven off a pulley
from the crankshaft (or directly from a camshaft with unit injectors),
I can't see how that can happen unless an attempt is made to start the
engine.

I know in the case of my VW (indirect injection, mechanical pump) the
only fuel lift pump is part of the high pressure pump, so it would be
impossible for petrol contaminated fuel to move from the tank without
cranking the engine.

Whilst glowplugs may be linked to doors etc, it will have no effect on
a misfuelled diesel vehicle - so is this "don't touch it" an urban myth
or is there some other reason?

Maybe electric fuel lift pumps (that run indepedently of the engine for
some reason) on some diesel vehicles?