i began writing a simple plugin that forwards messages to a custom http service so that this service then can for example send some gcm messages or other crazy things.
The plugin calls a custom url and posts a json object that consists of a string for the sender jid and a string for the receiver jid, and a third string with the message body. (The java object: https://github.com/meisterfuu/Openfire-GCM/blob/master/src/java/de/meisterfuu/op enfire/gcm/EventObject.java)
At the moment it can forward every message or only the messages where the receiver is offline. Regex matching with the ressources is planned. Of course you could to more filtering in your http service if you want.
Contribution and feedback is welcome.