Hi,
In Zeroshell, one IP Address/Subnet is attached with one Interface. So I don’t know how is possible to subnet 192.168.1.x to get Internet in your architecture. May be DD-WRT Router make NAT ???
The simpliest way to provide captive portal to the subnet 192.168.1.x is to use a other interface for him.
EDIT :
I don’t testing this solution but i think if you implement 2 VLANs on your concerned interface, you can achieve your goal.