Hiya, I did a similar image to what you describe,  I did it by doing 2 seperate renders, one with the girl looking into the mirror and her reflection then I did another render with the demon morph and textures applied.  I then took both images into PSP (it can be done in PS too, or any other graphics program you use) and l layered the renders together and used the eraser to erase the relection/ and girl I didn't want in the final image.

 http://www.renderosity.com/viewed.ez?galleryid=1047930

Hope this helps.