that would be quite simple in general. there may well be template configs out there for that use, but essentially it would be a default installation that happens to have no local delivery. if you do percieve prolonged outages you should be able to modify the retry and expiry intervals for it's relaying to other servers. just set the server up to relay for the end domain, and pretty much ignore the rest. i've not touched postfix before, but this doc looks nice and concise (yay google...)
http://www.akadia.com/services/postfix_mta.html particuylary the "Mail transport switch (Mail Proxy, Mail Gateway)" section.