Two data points that differ from the developing wisdom here. In late October I knew I would be checking 2 bags on a simple, standalone UK domestic ticket purchased mid-October after the rule change. I saw what it said on the BA website, but I know that in various contexts OW carriers often just forget to mention OW equivalents. So I asked Nicci (official lurker on BA board) whether the extra baggage allowance applied to OW Emeralds and Sapphires. Her answer: Yes, provided the FF number associated with the booking is the one that has the elite status. (I.e., if you are an AA EXP, your AAdvantage number must be on the booking.)

I tested this answer by checking two bags at EDI on 10/30. I paid no fee. The TA didn't even mention it.