I've done some digging. I found some enlightening information.
It doesn't specify if SU is allowed to collect additional fees for interline bags in transit, but it does say that USDOT rules don't compel them to honor upstream elite status waivers.
I'll edit this post if I find out more.
- According to AS, domestic interline transfers are honored under the first airline's policies, including FF waivers.
- Historically (at least until 2010), the first airline's policies applied when interlining
- As you mentioned, AS will grant an international carrier's baggage policies in excess of their own, whether interlining on one or multiple tickets.
- A standard "Multilateral Interline Traffic Agreement" exists under the form of IATA Resolution 780.
I didn't see any per-bag transit fees in the example agreement, but I may not have found the most recent revision.
- There was previously an interline baggage fee receipt designed to be presented to download carriers.
AS described it as an ancient handwritten red-carbon document (IATA Resolution 742?), but it appears that this has been rescinded and some other form of proof has been added to the Multilateral Interline Traffic Agreement.
https://www.iata.org/publications/Do...re-changes.pdf
- Frequent flier and class-of-service benefits from the first airline aren't required by USDOT to be honored for downline segments, although they usually are in practice.
- According to AS, you could prove additional downstream FF status and the first airline should be able to honor those baggage benefits.
- There are also bilateral and multilateral airline agreements that often honor these benefits anyway (e.g. AS+AA; alliances)
- At least with a single ticket, an airline is only permitted to assess additional fees at a stopover.
The rules are silent on if/how they would apply to an interlined bag across multiple tickets. Same DOT document, p. 40, Q. 45