I have a viewvc installation at http://www.example.com/cgi-bin/viewvc.cgi as its
root.The current implementation only allows setting the hostname and path, not
the script name. So when I use the above as url, the plugin sends me to
http://www.example.com/cgi-bin/viewvc.cgi/viewvc which doesn't exist.

I suggest that the configuration uses the full url and doesn't append "/viewvc".
Backwards compatibility might pose a problem, but that could be remedied by
appending "/viewvc" to the url on upgrading.