I've never seen a scan that did justice to foil. I imagine you could juice it up in photoshop by adding a semi-transparent layer of something like this:
Alternatively, I think you can get better and easier results with a camera and good lighting. Try to take the pic straight on, or be able to flatten out perspective in post. Try with lots of light, try with and without a flash, try to bounce the flash, etc, and see what you get. I would take a pic of the art only and photoshop it back in a case.
I have a few games with foil covers or slipcovers. Let us know how you do and I might try my hand at it too.