Awesome strategy. When first starting out a few years ago I tried something similar at the bank. They busted me right away on it, and I deserved the 'talking to'...looked a lot like fishing for customer account numbers. So that was one time they were right and I was wrong.
Speaking of ethical things, try this on -
A lady is funneling her change into the machine with one hand, while balancing her 1 year-old daughter on the countertop with the other. It was slooow going. Major delay in morsecode's Saturday run, ok?
My finely tuned ears hear a silver coin plink into the reject bin down below as the lady is wrapping it up. All is quiet as she presses the 'End' button (here comes the probably-questionable-but-I-sleep-just-fine part)...Timed perfectly to drown out Penny saying "Be sure to check the reject box for valuables blah, blah, blah...", I gush - "Such a cute baby! Look at those dimples! How old is she?"
It works as planned. The lady thanks me for the compliment, slings the brat onto her hip and marches off to the teller line. Cha-ching...one more Mercury goes into the stash.