Not sure if this is intentional or not, but I commonly log into my local web instance instead of going out to plex.tv to keep all the traffic internal to the network. What's odd, is when I log in to my local machine the Chromecast doesn't show up. Example:
192.168.0.3:32400/web/index.html#!/dashboard -> Can cast to rasplex
The current implementation of the Chromecast controller in the web app requires it to be a hosted application.
However, if you do browse your media on plex.tv and cast then the Chromecast will still communicate to PMS totally within your network (assuming the CC and PMS are on the same network, of course).
I can’t see the chromecast in the webui both if I connect locally or trough plex.tv. This would be a very nice feature, now I always need to grab my phone to start a movie