I tried passing the timezone configuration 'tz=Europe/Berlin' in the parameter 
'DEFAULT_BOOTOPTIONS' in '/etc/grml/grml-live.local'. The '/' symbol causes this 
error:

    sed: -e expression #1, char 81: unknown option to `s'

A look into the grml-live script shows, that 'DEFAULT_BOOTOPTIONS' is used inside 
sed in an 's/.../.../' statement. It'd be a good idea to either make the 
configuration variables sed-clean or document this problem in the configuration 
file.

I solved the problem by using 'tz=Europe\/Berlin'.