Thanks Colin, the problem is the customer has flat network. They need 1 group of clients on one SSID to get a DHCP address from their network and anther set of client to get an address with a different gateway for their traffic is set to the Internet through a Censornet web filter. This would be very easy if they implemented VLAN's.
We tried another setup on the IAP DHCP server, this seamed to work but then we found out the IAP was also giving out addresses to wired clients.
I think the only option is to VLAN the network but it is quite large.