From what I understand of your request, you don't want to extend or manipulate the image, just size it correctly when you crop it to account for the edge wrap. Instead of using the stretcher size, you should just set the crop tool to your total canvas size including borders. Then, you will not have to do anything at all, post crop. I use 2" borders for 3/4" deep stretchers, so I would set the crop tool to 24" x 34" for a 20" x 30" canvas. For the deeper stretchers I use, Fredrix Gallery Wrap bars that are 1 3/8", I use 2 1/2" borders. I would crop to 25" x 35" for the same size canvas with the deeper bars. Is that what you were asking for help with?
FWIW, I do most of my printing for artists (and a few photographers), and used to use PS actions, but have switched to Perfect Resize exclusively for my gallery wraps. Genuine Fractals sucked at it, but they made it easy with the current version (with the lame new name). I wind up often using content aware fill to avoid repeated artist signatures or odd edge artifacts, and sometimes get into more elaborate cloning and copying when a face or other detail is too close to the edge and looks stupid when mirrored.
I'd find simple images that have enough original content to use that as my bleed to be a piece of cake. Hard to get much simpler than that, and much more forgiving to stretch, too. I know that some people have elaborate methods of poking a pinhole in the corner, etc. to register gallery wraps. I just have templates for the sizes I use and register everything from the trimmed canvas edge, which I always keep consistent. It is really easy that way. I have one template for the corner registration, and one for trimming the excess canvas in the folded corner.
