Routing based on dport

I have a system with connections to two ISPs, meaning two NICs
connecting to two boundary routers, each of which could be used as a
default route.

For various reasons I really want to force packets out one interface or
the other based on the destination port. The discussion of why is long
and the short form is "it fits the client's needs."

Any clever solutions? Firewall is a 2.4.recent kernel, probably RH9.0
stock unless there's a really good reason to use something else.

Oh, and a third connection will be added in the next few months, again
for various valid but non-technical reasons.

--
Bill Davidsen <(E-Mail Removed)> CTO, TMR Associates
As we enjoy great advantages from inventions of others, we should be
glad of an opportunity to serve others by any invention of ours; and
this we should do freely and generously.
-Benjamin Franklin (who would have liked open source)