For some added context, it's worth noting that X-Plane is multi-platform software and already uses OpenGL, too - so the corresponding shaders are indeed GLSL.
Also, the code is distributed as freeware under the LGPL:
https://github.com/FarukEroglu2048/Enha ... er/LICENSEHowever, the standalone shaders I found are rather trivial actually (under 50 lines of code apparently?)
Most of the meat seems to be inside the cpp sources and textures themselves.
Looking at the sources, they're already using mostly plain OpenGL/GLSL calls, so people interested in using those sources in FlightGear would first of all want to port those to use OSG instead, and add everything to a standalone osg example - at that point it will be much more straightforward to tell if/how this could be integrated into FlightGear.