Thanks.
I think I finally completed my regex
to forward empty url parts, php files and slugs (wordpress) to the reverseproxy.
And the rest served via Hiawatha (.js, .css etc)
It works with the default posix regex, so that's great.
ReverseProxy (^\/$|^\/.*\.php.*$|^\/([a-z0-9-]+\/?)*$) http://127.0.0.1
If you have any tips or improvements, I'll be happy to hear them.