1. Make your 3D window and Photorendering window the exact same size in Pixels.
2. Set your photo rendering window to generate an image quickly. (your not going to used it) (you might just do flat shading with the internal engine)
3. Create an Epix file. (save as from the 3D window, it will render the photorendering and write the Epix data into the image)
4. Do another save as from the 3D window as an image file say .tif. (The Epix file is a Tif with extra data for Piranesi)
5. Open up Piranesi and your Epix file from step 3. and then just use the image file from step 4 to paint over the entire image.
This way you still have the Piranesi Depth and Material info but with a new starting point in terms of the RGB.
You can find additional Piranesi info
HERE
Including the paper I presented at ArchiCAD University on Piranesi.
David Pacifico, RA
AC27 iMac i9, 32 gig Ram, 8 gig video Ram