for GUI, it depend what quind of GUI,
it's possible to do simple GUI with script with brutal force > linking GUI to camera

Or it's using the c++ SDK (Game plugin, see WaterGameDemo) where you can draw a gui layer (wish is in a different scene with an ortho camera for example).

Render to texture is implemented, you can do it with c++ SDK.
For video, it's already possible to use a 3rd party lib and send the video to a texture.
With libtheora for exemple : http://www.theora.org/

In the future I'll try to implement lib theora by default.