Goto Video Options, and select Default OpenGL mode. It WILL fail. After doing this, Quake2 will recognize gl commands. Goto Video Options again, and choose Default OpenGL mode. Make the resolution be 320x240, reduce the screensize to the smallest possible, and set fullscreen to 'no'.
Press escape or enter, to save the settings.
Load a map. I have a p133, and the frame rate was around 1fps. It was decent enough to see if the colored lighting looked ok or not. Every once in a while, QBSP3 will decide to merge brushes that it shouldn't, and you get the stray one-sided polygon effect. There is a very simple solution to this.