I just implemented XEP-0049 - Private XML Storage for my Jabber client (source here). What it does is allow a client to store arbitrary XML data in a given namespace and retrieve it using the same.

To demonstrate a creative usage of it, I added an 'onconnect' event that will run any Javascript in 'soashable:userscript:event:signon'. Maybe stupid or dangerous; I haven't considered the implications carefully; but it's just a proof of concept.