As a BA Gold (plus CX DM and QF Plat, although it shouldn't matter)...I was also just refused access to the BA Lounge at BOS while flying CX (in paid J). The reason is, "the lounge becomes a CX lounge only at 21:45". I said, I'm not trying to access the "CX lounge", I'm accessing the "British Airways Lounge" during its official opening hours ("Daily: 05:00 - 08:00, 10:00 - 13:00, 14:30 - 01:00").

It's literally...BA Gold refused access to BA Lounge while flying oneworld. nothing related to capacity - the lounge was at most less than 50% full when I looked...