Hi Adam,
in the SSID Access Page you can use two way to tunnel the Trafic to a way of concentrator:
NAT mode: Use Meraki DHCP this will tunnel trafic and use DHCP from Meraki Cloud.
VPN: tunnel data to a concentrator this is the MX Firewall
but both solution are sending the Trafic to the Internet or at least to the Interface on the MX holding the Public IP.
We had the same Problem with Apple TV device on the Meetinroom Screen. If you put them in the internal Network to share your Laptop Screen, Guest will not be able to share there Screen true the Guest WLAN. So we put the Apple TV on a VLAN called "Apple TV" and now we did the Access Rueles on the Meraki MX Firewall. so you can access the Apple TV from the internal Network and from the Guest Network without problems.
Hope this helps
Regards Michel