Oh I see, makes sense. Although, if it's transparent and it subtracts 'too much', but not enough to lose the structure of the image, you could probably fix it up via levels/adjustments, depending on your needs. I doubt it would for a portal from portal, though. Sounds like the best thing I can do is put a portal (for example) on as smooth of a surface as I can, then use the defringing techniques you linked.

Very helpful, thanks! I was also wondering about another technique (whether it is possible and if it would yield good results): If you can get an image of the game with and without the target artwork, you could subtract one image from the other. Like in Portal -- get a closeup of a wall, take a screenshot, then add the portal, and take another (without moving at all). Would that be effective?