I have coded some stuff, not the way it is discussed at the moment. I started much sooner coding a plugin API.
Test Server: http://www.roland-liebl.de/webmail
user:
demo@liebl.ath.cx
pass: demo
Download a complete package (based on SVN 1711) @ http://www.roland-liebl.de/dl/roundcube/
Even if you don't like the API please see the features I have implemented: http://www.roundcubeforum.net/showthread.php?t=3389
Most things are worth at least a review. Maybe you want to adapt things which are not hmailserver specific into default RoundCube.
-Roland