A few more data points. Despite having been denied access back in April as reported above, I have subsequently been allowed access to T3 with my T5 boarding pass on every other occasion during my connections. Usually I have to explain what I am doing and then I'm let through, but there was one agent who after scanning and got the error message, simply asked "going to the Cathay lounge?". She must have had a few Flyertalkers pass through on her shifts!