Why not BWI?
Seems to me if you're going to be stuck at the airport that long, why not just take a BOS-BWI flight (Jetblue/Airtran/Southwest all have large amounts of non-stops), and if it's a weekday, hop on the Amtrak or MARC commuter train from BWI to DC (weekends I guess you'd have to take the taxi/bus to the Greenbelt metro). Or just do a one-way car rental from BWI to DCA (which is usually treated as a co-location for most rentals).