The documentation outlines HLS and MPEG-Dash (among others) that are supported. From an Android sender, I'd like to stream a low latency protocol like WebRTC or RTP, could a custom reciever be made to support this?
Receiver applications are just HTML pages with a script running, so yes it was possible to get a WebRTC client on the Chromecast. It's relatively straightforward to set one up using socket.io, and latency is good. Performance is limited however for some reason, anything high resolution (let alone 4k) is unwatchable.