Okay, last update on this. There's a lot of information on the Google dev page for Chromecast. The most interesting thing to me is how receivers work. Basically, they're just web applications (HTML, JavaScript, etc.) and you get a limited list of codecs. Notably, a receiver app needs to be "whitelisted". This isn't really an open system, despite being built on open standards. It is very simple, though, which is at least a good thing.

One thing is now abundantly clear: given the codec list, there's no way for a TiVo to directly stream its records (in standard MPEG2) to a Chromecast. You'd need to insert a real computer somewhere in the loop to run a transcoder. The whitelisting structure hopefully doesn't impose a huge impediment to building a player for content you've got saved on a server elsewhere in your house. The "receiver" would be a whitelisted web app, but it could then stream from your home server as well as anywhere else.