After a few days of hacking the sourcecode I am proud to provide you all with a shorewall6 module for webmin.
This module (rev 1) is included in the main development stream of webmin. The current version (rev 2) has been submitted moments ago.
The development takes place using webmin and can be found here: https://github.com/woutervb/webmin.
But obviously one can also wait for the next webmin release to appear or use the main development branch from webmin itself (https://github.com/webmin/webmin).
I hope you all enjoy this piece of code and start building ipv6 firewalls
(The code has only been tested om ubuntu 12.04 lts, but since it is based on the original shorewall webmin module not to many problems are expected)