If you really need to get under the skin of jabber.el, you can add
functions to the lists jabber-message-chain,
jabber-iq-chain and jabber-presence-chain. Normally
those lists contain only the functions jabber.el needs to do its job,
i.e. jabber-process-message, jabber-process-iq and
jabber-process-presence, respectively.
The functions in these lists will be called in order when an XML stanza of the corresponding type arrives, with the entire XML stanza passed as the only argument. Earlier functions can modify the stanza to change the behaviour of downstream functions.