Thanks for the suggestion!
I have given the problem some more thought, and I’'m pretty sure that I could do something like this:
Create some kind of Wildfire plugin that publish IP-address of client sessions, lets say on a webpage.
In my client (using Smack) I check this webpage for each user thus getting the IP from client sessions.
However I hope (and suspect) that there is some smarter way to accomplish this: Maybe some extension on the server that could be used directly by Smack (with some kind of IQ thingy). I’'m not an XMPP/Wildfire/Smack expert though… Is there such a solution", and if so: I would much appreciate some pointers in the right direction?