If it is piston slap the noise might be reduced by raising the idle speed slightly, say 25 to 50 rpm. Also, I would agree with them that it is very unlikey to cause any future problems.

However this noise matches all the symptoms of an small exhaust leak of some sort in the head area. Having documented your complaint with the dealer, I would suggest living with it for a while and seeing if it gets any worst.

Was this Pacific or Mossy you were dealing with?