Background: My company is moving into new offices and we would like to control all of the LCD TVs on our walls (around 50 screens) using Chromecast Ultra devices, such that each will display a different webpage. So that's 50 Chromecasts that will be connected via Ethernet to our local network.
I reviewed the Cast SDK documentation on how to build a custom receiver application, but to my understanding this is for sending a message to a single device every time. I wish to build a simple control panel (can be either a web app or an Android app), where I can select a number of Chromecast devices and send all of them a message (with a URL to load).
Can a message be sent to multiple Chromecast devices at once, without having to send the message to each device manually?
No; using the Cast SDK, you have to send a message to each device manually by establishing a connection to that device and sending your message. Alternatively, your receiver running on your chromecast devices can communicate with your backend server and you can use a PUSH mechanism to send messages to your receivers from your server; that, however, is outside of the Cast SDK.