example of custom glsl on particle effects:
FTE uses a shader with the name "lightpass" for its rtlights. Use $diffuse or $specular or $normalmap as a texture to use the surface's regular textures.
FTE's shader system does not have the same single-pass limitation as DP's shader system. Part of this means that it does not have a single master glsl file. However, glsl effects are fully replaceable on any individual shader - and everything is drawn with a shader in FTE. Know the name and you can replace it.
Frankly, a couple of things are still subject to change, like the default shaders that are currently generated. It is possible to name a glsl script in a separate file, but the syntax is going to change because the current stuff breaks horribly when using glsl shaders at the same time as d3d.