Just got my question answered by a SBSC Nework Engineer (Richard):
The SA520 only has one DHCP scope, as you mentioned. As a result, the
VLAN's are all port based VLAN's.
The steps you want to take to add your wireless SSID's to your vlan's is as follows:
1) Under Wireless - Profiles - Create your SSID's.
2) Under Networking - VLAN - Port VLAN - You should see your
wireless SSID's listed here under your wired VLAN's.
3) Under Port VLAN, Select the SSID that you want to add to VLAN's
and select EDIT. - Add VLAN's to that SSID. In order to select
multiple VLAN's for a given SSID, you must set it to either General or Trunk port.
Tested by me and it works perfect.
Robert