I'm a little late on this response, but I feel like I can provide some good insight here, as I spent several years up there and I've run into this exact problem on that AA morning flight to LHR when coming down from the Hanover area a number of times. I agree with the other posts, as it might be too risky to make the transfer that morning. My advice would be to take the Dartmouth Coach down to Logan the night before and take what you saved from not paying for the cape ticket and just put it towards a cy, fi, etc down at the airport for the night.

If you can find a stay+park deal at one of those hotels you might even be able to just take your car down.

One last piece of advice. The Dartmouth Coach doesn't have that many runs during the day (as I'm sure you know), so be sure to check that your return arrival time gives you enough time to make the last bus. (assuming you weren't able to find a stay+park deal at a hotel) I noticed this conflict in the past with the tatl AA flights and I have a number of friends who have been left to find their own bay back up to the mountains of nh.