I don't think that this is under the control of JBoss AS. It just sends something out and it is the TCP/IP stack that determines how the data will be routed (that is, via which address). I don't know how, or if, you can configure the TCP/IP stack to send via a particular address. I have often wondered this myself but have not yet been curious enough to google the issue.
On thing you can try is use tracert (or similar utility) to trace the route to the destination IP address - this should tell you which IP address on your machine is the starting point. I don't know if you can tell tracert which IP address to start with. But it sounds like some fun experimenting to do on a cold rainy day (that is, if I already did not have a ton of work to do).
By the way, if you learn anything, please post it here.