Originally Posted by
number_6
For completeness, the baggage allowance of the longest leg applies to all other legs on all airlines (under the IATA MSC rule); so it doesn't matter that it is a QF codeshare or on a Oneworld airline; you';d get the same flyihng DL ATL-DFW connecting to QF if on the same ticket, for example.
I don't know the answer to the specific question asked by the OP, but are you sure that this general description of the IATA rule is correct? I thought that it was based on criteria other than the lengths of the flights. While it is often the case that the Most Significant Carrier is the airline operating the longest sector of the journey, that is not necessarily so because the rule is actually based on criteria of first crossing of IATA zone or sub-zone boundaries.
I also thought that there was an exception for journeys departing from US points (or perhaps for US airlines), which by DoT regulations have to apply some variant of the IATA rule. But that is a hazy memory in the back of my mind, so it may be wrong.