Source address routing policy defined in a System V way

[Bob Richmond]

Ok, so this seems like it should be a fairly common configuration and
there's no method I can see to define it entirely via System V
configuration files. Say you have two network interfaces on two
different networks and with two different default routes. This can be
used, for instance, if you had two different ISPs and wanted to
provide the same services to both links from one server. You can
set up two routing tables for each default route, and set up a rule
that any packets originating from a particular network must be
responded to through the same corresponding gateway:

ip rule add from 192.168.0.0/24 lookup 100 pref 100
ip route add table 100 default via 192.168.0.1 dev eth1

Now, being able to define a GATEWAY for each interface definition
(eg: ifcfg-eth0) IMPLIES (at least in my mind) that you can define
a different default route to use to respond to packets received
from a particular interface. This, however, is not the case. :)
I'm not sure how this variable is typically meant to be used.

Can this be made into a new feature, or am I going about this
all wrong?