Can anyone speak to this? If it works, it can essentially mean almost unlimited miles. But I assume BookingBuddy would receive some kind of notice that the flight was cancelled, even if done with the airline.

This is somewhat OT, but what if you did do this, managed to get your US miles quickly, then used all of them up in a booking before Booking Buddy could get a hold of you? I realize with something like Amex MR points, the credit card company could ding you for the points, but could an airline send you a bill for refunded miles?