What a great idea! You could use water activated invisible ink to tattoo the QR on your scrotum. Then if someone tries to bum money while you're sunbathing you can tell him to lick your balls.
That's folly sir, folly I say. The heat of the sunlight would cause you to sweat, thereby activating the ink laid upon your scrotum! All folks who thereupon gain sight of it could steal your coins!
ROFL - you're rationalizing a scro tat.
No, she's just overlooked an obvious flaw. As someone with first-hand experience with scrota*, I can tell you that the surface curvature and wrinklage vary greatly from day to day and even from minute to minute depending on a wide variety of factors, such as posture, ambient temperature, the angle of the dangle, and "shrinkage". It's simply impossible for a QR code to be made readable under all circumstances.
*That is, scrota in general, rather than the specific scrota of multiple individuals. Not that there's anything wrong with that.