Setup wireless bridge with DHCP?

You pull in to KOA and they give you a ticket with the password for 1 device. I setup my OpenWRT router up as a wifi client via the KOA captive portal.

My OpenWRT router then acts as a DHCP and DNS server for any devices which connect to MY router via LAN or wireless.

I have done this before with DD-WRT and Tomato but for the life of me cannot figure out how to make this work with OpenWRT.

I can get the wifi relay to work where I'm getting IP addresses from the main DHCP server using relayd but if the DHCP server on my router is enabled it will not work. I want to hide behind the NAT firewall and serve my own IP's because a captive portal is only going to see the initial MAC address of my wifi router and not any of the MAC addresses of my phone, laptop, TV, etc.

See attach network diagram:

I think you already know what you want but just haven't hit the right search term.
You want a "wireless client" rather than a "wireless bridge"

I'm not sure how you deal with the captive portal bit on the wifi side. There is the Travelmate package which has some capability to do this which might inspire you or you might be able to just use it.