I'm hoping that the Plex developers see this thread!
I've been doing research into Chromecast recently, both for personal experience and because of work projects. I've ported a DASH JavaScript player to Chromecast with very little issues, which shows promise for video playback.
I know that Plex already transcodes videos and serves them up as HLS m3u8 files. DASH isn't all that far off from HLS, especially if the Plex server is transcoding to mp4 files before creating the TS chunks for the HLS. If the Plex server could output DASH manifest files the Plex team should be able to make a Chromecast DASH receiver application without many issues. It will be especially easy if the transcodes are mp4, since DASH supports segmented streaming from solid mp4 files provided that byte range requests are allowed.
The Dash.JS project is an open source JavaScript player for DASH content. I've already managed to create a Chromecast receiver application that plays DASH content using Dash.JS. VOD support is pretty solid, provided that the mp4s are supported by Chrome.
One thing that is of particular interest to me is the support for multiple audio tracks and subtitles. I have a lot of anime on my server, so it's very important that these features work. The Dash.JS project doesn't yet support these features, but multiple audio tracks are a stone's throw away and somebody is already researching subtitle support.
I've done a lot of the development on the Dash.JS player and I'd love to be able help the Plex team in any way to get the Chromecast support off the ground. Plex is the primary reason I got the Chromecast!
