In this case, the MCT would have less to do with D-I and more to do with T3-T5 since outbound international flights don't require any additional processing besides a document check upon boarding.

I agree 55 minutes is tight but doable if your flight is on time. The shuttle is indeed only for BA customers, see this thread on the topic here:

http://www.flyertalk.com/forum/ameri...nly-t3-t5.html

Head straight for the exit doors of T3 on the concourse level; don't go down the escalators to baggage claim. Up the stairs or elevator to the ATS, which runs every few minutes, and only takes 2-3 minutes to get to T5. Security lines at that point in the evening shouldn't be too bad since most of the evening rush at T5 has already departed at that point.

Also, try tweeting AA and briefly explaining that while it's a legal connection, having to exit the terminal and reclear security means it'll be tight. They may be able to open up space in F on an earlier flight.