I've posted the libRocket patch I was talking about earlier at codepad:
http://pastebin.com/N15H8jsZ. In summary, the changes were:
* Added a new RocketGUIManager class which can be used to implement a libRocket GUI in FIFE. To use, simply create an instance and pass it to FIFE::Engine::setGuiManager (works from Python, too).
* Added a new demo program, rocket_demo, to demonstrate the new libRocket GUI.
* Updated the build/<platform>-config.py files to look for RocketCore, RocketControls and RocketDebugger.
* Modified FIFE::Engine::setGuiManager to automatically register the new GUI manager with the SDL event system and clean up the previous GUI Manager.
* Modified the Python bindings for FIFE::Engine::setGuiManager so that it automatically assigns ownership of the new GUI Manager to the Engine.
* Updated the Python GUIChan extensions to utilize the new behavior of the FIFE::Engine::setGuiManager method.
* Added "castTo" static functions to the Python bindings for GUIChanManager and RocketGUIManager which downcast an IGUIManager pointer from Engine::getGuiManager, allowing Python code to utilize these IGUIManager pointers.
The last couple of commits to fife-0.3.3dev broke my rocket_demo.py code somehow and it raises a RuntimeError when it tries to set the default font for the GuiChanManager. This may have something to do with the fact that I didn't bother to stop GUIChan initialization in ApplicationBase's __init__ method for the demo program, but simply clean up GUIChan and replace it with a RocketGUIManager instance. Regardless, I don't know enough about the changes to debug it so you'll have to take a look.
I couldn't figure out how to interface libRocket with the RenderBackend, specifically to render textures. So I just copied the OpenGL renderer from the libRocket samples "shell" framework. It seems to work fine within fife, though this might not be portable.
I should note that I haven't tested this on Windows or Mac.
Let me know if you have any questions.