Re: Ah, you didn't mention that bit.
Yeah, it can be done without the QR code via the camera, similar to watermarking current images/video (IIRC some games/software does do this to catch cheats uploading video to Youtube).
However, as you say, the codes getting out would be oh too easy and the computation rather large I assume (Bitcoin for example is starting to hit economically unviable levels, right?).