you could do something like :
YOUR ARCH BOX ::
iptables -t nat -A POSTROUTING -o tun0 -j MASQUERADE
iptables -A FORWARD -i tap0 -o eth0 -m state –state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth0 -o tun0 -j ACCEPT
Then everything at your dorm that is using the arch box for a gateway will then get routed over the vpn. Is that what ya want? As long as you get the arch box vpn client going the above should work.