by frag.machine » Thu May 27, 2010 4:20 pm
Build your map in exactly the same way you would do with WinQuake/GLQuake (that's it, using brushes with a regular Quake sky texture to seal the map hull). When loaded in regular Quake, your map will show the selected sky texture, but when loaded in any engine with skyboxes enabled, this sky texture is not rendered; instead, it will render a box with 8192 units wide, made up of 6 images (usually, 128x128 pixels TGA,PCX or JPEG - check the engine instructions) named <something>up, <something>dn, <something>lf, <something>rt, <something>ft, <something>bk, around your level, creating the illusion of a distant background scenario. But you still need to seal completely your map, just like before.
I know FrikaC made a cgi-bin version of the quakec interpreter once and wrote part of his website in QuakeC 
(LordHavoc)