The steps you mentioned in your original post will work. Follow the steps here http://community.cambiumnetworks.com/t5/cnPilot-E-Series-Hotspot-E500/Run-an-cnPilot-E-series-AP-as-a-router/td-p/54393 to assign the clients to a new VLAN (say, VLAN 30) - create an interface VLAN 30 on the AP and assign it a static IP address, enable NAT on it, and create a DHCP pool for VLAN 30. This will be your local subnet.
Assuming you have created a VLAN 20 on the AP and there is a gateway on this VLAN that can route to the general Internet, the AP will NAT all wireless client traffic and hide it behind its VLAN20 IP address before routing it to the gateway.
-Trevor