I know Open GL shader calls are being used to add new UI elements (LazyWizards Combat Radar)
Not
shader calls, unless he's done something new since the last time he made a release; using a shader requires access to loading, compiling and invoking GLSL fragment / pixel shaders. He's just using straight OpenGL, which is not the same thing.
Not sure that the game supports that atm, but maybe GLSL could get read in via JSON or something.
To do that kind of Fresnel stuff you're talking about, though, is fairly complicated (there are example shaders for that, but it's non-trivial to implement), and in the case of this engine, would need to be done with knowledge of what the engine is doing internally in terms of depth (you need to render this below the UI).
Anyhow, shader support would be cool, no doubt, for a few things at least. But Alex has said repeatedly that that is not a priority, and I'm not sure if we're able to access that part of LWJGL or not. LWJGL is certainly capable of dealing with GLSL shaders, so it may be possible; if yeah, then I might be able to do a few things at some later point.