What licenses / permits are required to perform slingload operations in a helicopter?
This one I found in the FAA Helicopter Instructor's Handbook

In the United States, FAR Part 133 covers this.  For the Flight Crew, you are required to obtain either a commercial or airline transport pilot certificate - rotorcraft helicopter with additional ratings for external load carriage.  This rating can be obtained only at the behest of the chief pilot licensed for external loads and under the employ of an operator holding an external loads certificate and requires both a knowledge test and a practical test as prescribed in Part 133.

The helicopter in question must hold a valid restricted category airworthiness certificate and must have been designated for the carriage of external loads on its type certificate or by STC per Parts  21, 27 and 29.

The operator must hold a current Rotorcraft Helicopter External Load operator's certificate which must be renewed every 24 calendar months.