Originally Posted by
pikapiu
Actually I just found something interesting on BA.com it says : If every flight in your journey has a BA code (e.g. BA1234) you will get British Airways’ checked baggage allowance, even if some or all of the flights are operated by other airlines (so-called 'codeshare' flights).
The problem here is such references do not (usually) deal with status-based extra baggage allowance. It is just dealing with normal allowances based on booking carriers.
You could risk taking extra baggage if you are prepared to pay the potential extra baggage fee (which is in fact not too bad on JL) if it turns out that the allowances are as per the lower limit, but if you would rather avoid that, then I'd really recommend keeping it within the standard JL allowance.