As of Tuesday at least the Flight Connection route from by the AA lounge, and the double door, was still there, so I doubt the return route has been blocked off, HAL do allow - and to some degree encourage - people to swap terminals for specific reasons. If AA is being used as an effective paid-for lounge, like the Priority Pass lounges, then that's something HAL are encouraging.
To summarise my current understanding:
- You can't enter T3 with a T5 boarding pass if starting in London, starting from landside. You have to go to T5.
- You can enter T3 without a T3 boarding pass for shopping and lounge access, if you are a connecting passenger, ditto T2 and T4.
- If your next flight is from T5 and you want to go T3, you best pass to T3 immediately after landing, following the airside bus transfer route via Flight Connections.
- You can then return from T3 to T5 by heading to the AA lounge entry area and you will then see a sign for Flight Connections and a route back to the buses.
- If you are already airside in T5 and have done security, it's a messy and non trivial process to switch terminals.
- If you are really departing on a T5 flight you can't assume a T5 boarding pass will be accepted at a oneworld lounge (though this case, involving AA Club membership should be OK)
- Despite the caveat above, I'm not aware of anyone being refused lounge entry yet but obviously it would be good to have specific data points.