Well, the biggest problem is that the textures are bilinear filtered. If you want to do a 3D Interface use Power Render. If you want to do a "simple" 2D Interface I would use DirectDraw.
You can initialize and start Power Render as normal. Then, set the viewport to 0,0,0,0 or if you want a 3D preview, set it to the position in the interface.
The rest of the screen is a DirectDraw Surface (I think it's wgtbackbuffer or so), which you can modify. Check out winutil.c (function resize). There you will find all of Power Render internals regarding DirectX.
Before I went to MFC to code my PRO-Editor I used the same method to do all my windows and button stuff.
