Yes, I have been boarded by bus from A10 onto a long-haul flight in First. Not a great departure experience if you get to the gate early for 'priority' boarding (since you end up waiting around on a bus rather than in your seat enjoying a drink). That said, if you know what A10 means (bus!) then it's very easy and convenient to sit in the lounge until close to departure and being last on. A10 is a very short hop down from Galleries First / Concorde.