I've got a similar question: I won't be able to take the last leg of a RWT and was thinking about getting a BP (without checking in any luggage), go to the lounge, enjoy some food, and then cancel my flight. Any problems? Will I get the miles?

Edited to add: obviously I wouldn't leave informing the lounge staff/airline too late. I'd probably go in well ahead of time thus allowing the extra seat to be sold if the flight is full.