If you look under "status benefits" on the One World website, it states:
Priority baggage handling
As an Emerald frequent flyer you will also receive priority baggage handling when travelling in any cabin on an eligible oneworld flight, giving your baggage priority handling from aircraft to carousel.. (Priority baggage handling is not available on flights operated by British Airways.)
However, even when flying in F or J, the tags make pretty much no difference. Don't even bother worrying about it. Don't ask me why BA are incapable of doing this. I'm sure if U2 or FR charged for this service, that they would achieve it 99% of the time.
https://www.oneworld.com/ffp/emerald-privileges