Hi,
No, atm the plugin system is sadly underdocumented. There was a general
design topic on the old wiki, but I can't find it anymore. It got lost
in the migration to google code I guess (@Zoran; is the old wiki still
available?). Creating a plugin shouldn't be to much trouble though. I
suggest you first take a look at the caldav plugin
<
http://webical-plugins.svn.sourceforge.net/viewvc/webical-plugins/trunk/caldav-plugin/>.
It uses maven as a build system and implements the same DAO's you would
have to implement to create an alternate backend. If you like you can
get access to the svn at sourceforge to develop your plugin, let me
know. If you have any specific questions just post to this list.
Hope that helps,
Ivo