choose IP for SNAT programatically

Hi!
I have a following problem.
There is a router with 5 assigned external IP's. The router uses SNAT for
these addresses.
I want to develop an TCP/IP socket oriented application working on this
router which task is to send some messages to servers located in external
network (internet).

.... and the problem is that I want to choose IP for SNAT from the
application.

Does anybody know how to do it? Is there any patch for iptables to provide
such functionality by for instance setting special socket option?

Regards,
Grzesiek