Wow...800+ posts and this is your first mileage run? Used to be you could do what you were talking about...just get your boarding passes all issued in advance and hand over the one you want to use right as you were going on the jetway. Those were the good old days. I once found a $58 RT Chicago-Omaha fare on DL that allowed connections in Atlanta (!), so it became an Atlanta-Omaha roundtrip. Some gate agents would even take boarding passes, tear off the coupon and stamp them at the connecting gate (if they did that you could get credit for segments not flown). Those days are long gone.