Are there any known instances of airline pilots passing off aerobatics as “turbulence”?
Recent discussion with friends who work or worked for one  major US airline, say they never witnessed such a thing. 

One of them points out that on Navy P3 flights, they would do it to newbies. Not often, but sometimes to recalibrate newbies.

With respect to commercial flights, all pointed out that flight attendants are at high risk for injury, and would not tolerate such actions. All it takes is one person getting hurt. A neck injury from an unsecured passenger, or a burn from hot coffee happen often enough on flights with "natural" turbulence.  Creating turbulence could be career ending, and could have civil liability.  

The population of pilots I was dining with are all over 45, and tend to be rather risk adverse. One, who has been retired for quite some time (he's in his 80's) says that people joke about it, but in all his experience, he too has never witnessed "pilot induced turbulence."

I have a blow-up of that far-side at my desk, though...