In my experience AA will pass through YQ on BA metal; for example ticketing an xONEx out of South Africa, AA will add the fuel fine to an initial BA-operated segment such as CPT-LHR even if it's booked using an AA code. I assume it has to do with the AA/BA/IB joint venture agreement.

Where AA differs on YQ is that they don't charge it for other segments operated by non-BA carriers, whereas BA does if it can. Note this is extremely difficult to document because of the cone of silence imposed by the AA (and BA) rates people. Even the AA RTW desk agents can't (and/or won't) reveal a detailed accounting of the construction of YQ/YR amounts on the ticket. They're neither published fares nor are they taxes that would require disclosure, so instead they're treated as a black box from which numbers are pulled.