This works at T5. I once had a check-in agent insist on putting my BAEC# onto the BP to let me check a second bag, I wanted the flight to credit to Avios.com (some promo or other). Agent phoned supervisor, still insisted on it. "Fine", I said. Bags tagged, down the conveyor. Walked 10m down to another check-in desk, "please can you take my BAEC# out and put my Avios.com?" / "Of course, Sir". New BP, job done.