Hi,
So we have a separate SSID for our casting devices which are accessible from the corp network, but the same principal could work for you.
This SSID is set to Bridge mode with Layer 2 isolation disabled and set to allow all traffic for wireless clients accessing LAN. Our Corp SSID is also in bridge mode.
You could possibly give each streaming device a static IP and set a Layer 3 firewall rule on the guest network to allow communication to the destinations you require (IPs of each streaming device) and block all communication with anything else on your network. and a rule on the corp network to also allow communication with the IPs of the devices on the guest network but block everything else - Not sure how this would look.
Im not sure of the limitations of the walled garden but you could try allowing the IPs of the streaming devices? or have them in their own VLAN and allow connection to that IP range. not sure how well it would work as Ive never tried it.
Cheers
Chris