The version 1.4.0 of the broadcast plugin has been released. Main new feature is the ability to include the broadcast service in the roster as a contact. So allowed users can now include all@broadcast.myserver.com to their roster or even a specific group like for instance mygroup@broadcast.myserver.com. Full change log can be found here[/url].
Link fixed. AFAIK, there were 2 issues when checking if a user was allowed to broadcast messages to a group. 1) Full JIDs failed to be considered as valid group members when the bare JID was entered as the group member and 2) the JID was used to check if it was allowed to send messages when the string representation was the correct value to use.
BTW, last night while I was trying to get asleep I kept thinking about this plugin. So a new version might come out soon that includes disco support so users can use the disco browser to easily use the broadcast plugin.