Check out MythTV http://www.mythtv.org/ an open source, linux based DVR - this will do all of the low level nonsense for you. I suspect this does what you want - you can record the things you like and play them back. Now, I don't think you will have fancy channels etc so there will be some manual work, but most of the functionality is there. There's a whole community that talks about setting up HTPCs and once you have this open source stuff setup, you can then extend it to do what you like.
Actually I do use MythTV - and it is from it in part that the idea was born. Doing it manually (as I have been doing a little) is just too time consuming and complicated.
As you say it can be done in part by extending and rewriting existing DVR software - but to do it properly you need licenses with content providers etc. to provide on-demand content with which to build your channels.
It's a good niche for a startup I reckon (rather than just clone Hulu)
It's certainly an interesting idea that is related to the sea change in how people interact with media that has been ongoing for the last half a decade or so. As far as getting something for your own purposes, I think building something on top of MythTV is probably fastest - avoids the annoying reinventing the wheel stuff. I have shied (sp?) away from video work because it is inherently computationally and bandwidth expensive - so most of my personal work has focused on text based stuff, which is much cheaper to make scalable (:
