I made a very simple version for glquake that simply checks if the texture is called glass, then use the neccesasry calls to turn blending on, set the blending mode, call the actual DrawGLPoly function then reset the blend modes back to normal. Works alright for me, although some times entities don't show up on other sides of the glass brush (which the brush itself is an entity something like a func_wall) unless it's made static. But then you can walk through it and shoot through it
I only use it for small details though so it isn't much of a problem for my uses.