Here's a definitive answer (for now at least) on transfering after a Domestic flight
There is currently no way you can officially access the Terminal Departure lounge after arriving on a Domestic flight without first returning landside in the terminal and then coming back through security unless you have hand luggage only and a valid (not Internet check-in) boarding card for your onward flight. And you won't have one of those unless you've first cleared the domestic reclaim and then been to check-in.
This is because although you have the correct security clearance as you have said, the security officer at the end of the lounge tunnel cannot verify your boarding card for your "return" to the lounge and therefore have no process for doing so. They do not have a bar code reader for the Internet check-in printouts otherwise that could be acceptable, but still with a risk of the bar code not working and you having to return to check-in anyway. If the demand was there I guess BAA might put in a bar code reader on that desk, but that's unlikely given the current downturn in traffic.
You'd never be able to plan this, but if you somehow knew your onward flight was actually departing from a Domestic lounge gate then you could always stay in there if you had an Internet Boarding Card to get on your next flight.