It's really going to need a DIB engine and all sorts of other magic. I expect given the speed of the RDP protocol, even without the DIB engine, its still going to be faster than running the standard X11->VNC->RDP bridge just due to the removal of many layers of redirection.
How is it going to handle DIB drawing without a DIB engine? Is there some offscreen bitmap support in RDP which this driver could offload to like x11drv?