Yes There has previously been a ZRH charter.
Your correct about Cornwall being very busy, Germany is a huge market for Cornwall and considering that I can't see why a FRA or MUC wouldn't work, if on LH it would give connections over the LH network both regionally and internationally.
Germany currently is a huge untapped market for NQY with the exception of the weekly, seasonal DUS.
Also, poldark seems to be having the desired effect on cornish tourism if the spin is to be believed, that should help to increase domestic and international tourism to Cornwall. NQY needs to cash in on this.
cs