no need to answer, I found it out myself in the meantime. And it works great! Thank you very much for the hint!
To point it out for others with the same problem:
The path must be relative to the /srv/www/webXX/web folder (at least on my Suse 10.1 / ISPConfig 2.28 Server).
Another thing one needs to know is that it takes a bit of time for the changes in the Apache Directives field to take effect.