Soo, I have a bit of an update for posterity..

Rather than modifying source as it stands, it seems that ipilter can
do simple routing tasks after all. So even though the stock NetBSD
kernel doesn't have the ability or gumption to be able to answer on
arguably the "correct" interface, a simple rule in ipf.conf will
over-ride this and re-route packets out the correct interface. This
seems a kludge, but here's an example:

pass out quick on fxp1 to fxp0:10.x.x.x from 10.x.x.y/32 to any

...where 10.x.x.x is the "other" router and 10.x.x.y is the
multi-homed server's IP address.

:) Kind of neat-o.

ttyl,
marc