I just read about using the Quick Mask option for cutting out a render, and it seemed to work for me pretty well.
To get into Quick Mask mode you click the right hand rectangle with a circle inside it from the tools window. Then you brush the outline of the render in black. The colored areas will be selected when you leave Quick Mask mode. After you have your render outlined you use the Magic Wand to select everything inside the outline, the stuff you want for the render, then hit Alt-Backspace to fill. If you accidently colored an area that you don't want just brush in white over that area. Then leave Quick Mask mode by clicking the white rectangle with a circle in it, right next to the button used to get into Quick Mask. Now only the render is selected, so you can either move it to where you need it or invert selection to delete the background.
Still takes a lot of time, but it might work for you.