of what you're looking for is available -- it's outside the scope of HME, but it's part of HMO. The video-specific portion of HMO (also known as TiVoToGo/TiVoToComeBack/MRV) is not publicly documented. However, you should read the HMO specifications
to learn the foundations -- TiVo's docs only talk about photos and music, but the principles are the same. Then, to learn about video, you can try out the TiVo's built-in web server, and study the source code of pyTivo and/or similar programs -- it's all reverse-engineered, and I don't think anyone's written up a proper spec. But it's (IMHO) pretty easy to understand from the available material.
That will get you the ability to parse the Now Playing List and transfer videos to and from the TiVo. It will not allow you to control it in any other way -- you can't start playback, delete videos, etc.
There is a third interface, for Series 3/HD models only, that lets you emulate a remote control over the network. It's very crude, and provides almost no feedback; but in principle, you could use it to automate any task you could do from the remote. See the "TiVo UI Control via Telnet"
thread for more on that.
Also, "hacked" units can be controlled through more flexible software like TiVoWebPlus.