Really, how hard would it be to make an announcement a few minutes before boarding to the effect of,
"Good afternoon ladies and gentlemen. My name is X, and I'll be helping you board your flight today. Please pay attention to this announcement, as our boarding procedures have recently changed. We will be boarding the aircraft today by class of service and elite status. If you are seated in our First/Club cabin you will be invited to board first. Once our premium cabin passengers are onboard, we will invite BAEC Gold members and Oneworld Emeralds to board, followed by BAEC Silver and Oneworld Sapphire, and then BAEC Bronze and Oneworld Ruby. If you are not seated in our premium cabins, please remain seated until premium cabin passengers have boarded, and wait until your group is called. Once all premium cabin and elite status passengers have boarded, general boarding will begin through the economy class lane. Thank you for flying with us today."