Shower is no heavy metal!!! ;-)
But I had cleaned myselfes last year, too, but not at the showers but at the coldwater-rim outside of the washcamps- i think washing there is more than enough for a fest like wacken.. after some minutes you´ll smell like before and you are as dirty as before.... i just don´t need to pay for the few moments of beeing clean.
by the way - i have the advantage (???) of short hair - you can wash it at the rim, too.
maybe i´ll visit the swimming pool in wacken this year on friday morning - that´ll clean to and i think there is some metal party, too - because there will be some other mheads, too, i think... that could be fun.