Hey it's the birthday boy himself - many happy returns!
Yes I believe the pin 7 output is very roughly modulated (digitally in fact - hence the problem with harmonics) audio..
It doesn't surprise me that the DAC can't be re-purposed in this way without major headaches.. It would have been a cool aternative use of the DAC if it worked though. :) I think I need to learn how make my own basic filter as one of my first projects.
Thanks for taking the time between packing many Pi3s and partying to humour my wild idea :) Have a good one!