I think the orginial intent was to try to get a portable solution, if
not, it looks as if the very messy 'divert' (man divert) feature under
Free 2.2.5+ does essentially what you want
the dhcp solution is unlikely to help since the server attempts to
maintain a unique binding against the mac address, so its likely to
just return the existing address in the face of a new request (or
worse, put the old address back in the pool and return a new one)
it would seem that a truly portable scheme is very unlikely....my last
terribly hacky suggestion is to use a different ethernet encapsulation
than one normally used on the host and redirect it off the mac layer
(depending on host host facilities)