Tempting to make a system that captures publicly posted photos of boarding passes. Then email the poster a warning along with proof of the personal information that they made available. However, I suspect this could get one in trouble.
90% of what's present in the boarding pass barcode (PDF417) is visible in plaintext on the actual document. The other 10% is relatively meaningless without access to the airline's systems (perhaps with the exception of FF numbers which are sometimes redacted).
My understanding (without trying this myself) is that it's potentially possible to recover a traveler's email and home address.