The main downside with an approach like this that I can see would be that it would no longer interoperate with other endpoints using mDNS, which are still only monitoring for announcements on 224.0.0.251. However, in this case it does not seem relevant. I'm presuming that Chromecast is somewhat of a closed system: Google defines how the senders work and how the receivers work. So it would be trivial to modify the application such that senders and receivers use and expect service announcements on a group other than 224.0.0.251.