As you know, Quake is the coolest game, that humanity could did. It's authors, Id Software programmers, are the real wizards. They released Quake code under GPL licenese. It's simply groove :)
Personally I really love this game and have many own demos, which remind me about past furious battles and help to find tactical mistakes. Most of demos are about hour long. The desire to convert them into video is natural. You can watch it on any computer having video player, create your clips and movies, set your own music on it and many more. The ONLY existed way was very crude, awful and took too much disk space and time(is about 15Gb and 10hrs on my 450MHz AMD-K6-2). This method described in "The Recording FAQ" by Uwe Girlich. The main goal of it's method is to save one pcx file per frame and then put them together into video stream by any encoder. There were a lot of shortcomings, problems and you might forget about quality or time synchronization question. Anyway, I put it's implementation in /docs directory as old_way.tar.bz2. You can try it :)
The idea to hardcode mpeg writing in Quake source was born long time ago. But it seemed hard to me and I couldn't spare little time to take a look on it. A few days ago I got a cold and had to stay at home a few days, instead of work. Despite of high temperature I couldn't lay without work. So I started a new project with proud name 'dem2mpg' :-).