offline_message_hook for group chats

I've got an iOS app backed with ejabberd, with a small extension based on offline_message_hook that uses APNS (Apple Push Notification Service) to deliver push notifications of messages that are sent to offline users. This doesn't work out so well for group chats though, so I'm looking for a strategy that would do the same for offline users that are part of a group chat. Do MUCs even keep track of users that are part of the room but offline? Would I need to extend them in some way to keep track of this? Could I subtract the set of invited users from the set of online users to get the offline users? What hook should I be using to do this?

Any suggestions on this or advice on a better strategy are much appreciated!