For access purposes Greenwich lounge is considered a Flagship Lounge, so the day pass would not grant access. Standard practice for when the Admirals Club closes before a flight is basically "You're on your own", especially when the Admirals Club is in a completely different terminal from your flight. There have been stranger things that have occurred than a highly empathetic staff member making a discreet phone call to secure an exception... but I wouldn't count on it.