… but you answer your question: yeah, I’m sure you could do it. Anything you don’t see within the GL GUI can be handled via LuCI (GL GUI → Advanced Settings) or via SSH to edit conf if you’re like me & don’t care for OpenWrt’s UCI.
Hypothetically it could just be a matter of setting the Beryl to set the Guest SSID’s client MACs to force’em to only use a specific interface (eg: tethering). iptables is used & Beryl interfaces are defined @ /etc/config/network. This might give something for a starting point:
