I was just wondering if there was any chance that someone could hack the code on Netflix/Wii disc so that the Wii could read data from a MyMedia channel. That would substantially increase the usefulness of my Wii for watching video since I have a bunch of videos I've recorded on a Mythbuntu box (and transcoded) to watch on my Roku. Sound at all feasible?
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If your goal is to only control the music being streamed to the Roku via some external control without interfacing with some client running on the Roku, I don't think there's much that can be done. Roku gives the client very little control over the audio player itself. Once the url(s) to the song(s) is setup, the player controls all buffering. There's nothing in the API that allows any control over the cache. I'm sure a Roku client app could be written to periodically check with the server to see if the cache should be flushed, but then you'd have to add something to the SlimServer to respond to the Roku client.
On the other hand, the Roku client could be signaled somehow from the Roku remote control to flush its cache, but I get the feeling this isn't what you're looking for.