DD-WRT support DHCP relay and i was playing with this option also.
on pfsense i have denied unkown users to get a lease from dhcp and i have connected the pfsense LAN side to the campus network
DHCP forwarder is set on dd-wrt routers to forward dhcp requests to pfsense. routers are connected to campus network from their LAN side
Now when a registred wireless user connects to ddwrt router, it gets a dhcp from pfsense. but if it's not a registred user, it get an IP from the university DHCP after 1 minute : so no captive portal authetication and no security
and when a PC connects to campus network, sometimes it doesn't get its IP quickly. this is not a real problem, i think there are some additionnal setup to do with university network and routers.