I've actually got two scenarios to apply this:
Multiseat Desktop: two network connections both with internet gateways and two accounts doing bandwidth-intensive tasks on each. I want to split them up so one account only uses eth0 and the second account only uses eth1.
Server: I have two IPs on a server and I want to make sure the mail user only sends email from the second IP (eth0:1 alias)
The second can probably be IPTabled (I just don't know how) to route email traffic through that interface but the first will be dealing with all sorts of traffic so needs to be user-based. If there is a user-based solution, I could apply this in both places.