Im not quite sure what you guys are discussing, but I believe im using this feature. For instance in my compose_addressbook plugin:
JS:
// register the callback function
rcmail.addEventListener('plugin.compose_addressbook_receive', compose_addressbook_receive);
// register the callback function for the group expander
rcmail.addEventListener('plugin.compose_addressbook_receive_expand', compose_addressbook_receive_expand);
PHP:
// send the addressbook back to javascript
$rcmail->output->command('plugin.compose_addressbook_receive', array('addresses' => $contacts));
$rcmail->output->command('plugin.compose_addressbook_receive_expand', array('members' => array_unique($members), 'target' => $target));
Regards,
Cor
List info: http://lists.roundcube.net/dev/