Yes, you should be able to tunnel your Guest Network WLAN back to the SZ controller, and VLAN it off to either ISP, versus local break out (LBO) and drop client traffic for the corp net at the AP switchports. The SZ-100 has built in Dataplane, so just configure Tunneling under your Guest Access WLAN advanced options. We use RGE by default, no 3rd party equip required.
Can you maybe advise how to config the SZ100 physical port/vlan to hand off traffic? I could create the gre and ssid to use the gre but cant get the guest traffic to offload on second isp breakout on the sz100
In the WLAN go to config, then advanced and enter a VLAN other than 1 for the guest traffic. Then at the existing controller LAN switch port, make the guest VLAN a 'tagged' VLAN or a trunk allowed VLAN depending on the switch manufacturer you are using