I used a system at a recent hotel stay that used a randomly-generated code on the hotel TV's and a website you would visit on your device that would associate the TV with the phone. Worked pretty well. The site is cast2.tv and I haven't been able to find out more details about how the backend works or how much they charge hotels for the service.
The hotels I've stayed out with this functionality all had an Aruba Wireless system with a hospitality model AP on a 1-gang wall box somewhere near the floor or strapped to the back of the TV.
Aruba wireless (like many enterprise wireless vendors) has a mDNS proxying and filtering system which is essential for places like college campuses. Airplay and Miracast (infrastructure mode) both use mDNS for service discovery. In Aruba the system is called Airgroup. When seeing a new MAC address the wireless controllers queries the RADIUS server. The RADIUS server can return a configuration for that new MAC address which tells the controller what other devices can interact with this new device using mDNS.
I always assumed this vendor adds and removes Airgroup configuration to allow the phone and the TV to see mdns from each other while preventing this from other nearby devices on the same VLAN or SSID. This trick might be enough but also pushing a DACL to the wireless controller to properly packet filter all network traffic from unapproved devices would strengthen the solution.
