RTW type tickets cause a lot of confusion about traffic rights and codeshares. The bottom line is that legally you can take most AA codeshares around the world (a few countries are exceptions) provided that you have at least one AA flight in the US on the same ticket. The AA codeshare does not have to be contiguous with another AA flight. But many/most agents are not familiar with all the fine points and neither are all of the computer reservation systems. I guess for L tickets it becomes important, as L earns more on the AA codeshare than otherwise. I'm just glad that I don't every care when I am flying on the AA codeshare and when I am not, it will cause headaches on many reissues (depending on the country and computer system).

Finally, Q is never used for any OWE product. A/D will book into Y on one-class AA flights (instead of L), which is a great feature (basically allows you to overbook on those flights, they will bump someone on a lower fare for you). Otherwise A books into A or D or L (on 2 or 3 class flights), D books into D or L, and L books only into L (which can be a problem with no inventory on popular flights).

The woman at AA's RTW desk couldn't find the AY flight (or at least D availability on it), yet Expertflyer still shows D4 for the AY equivalent. Should I try calling AA's RTW desk again and ask? Can AY or BA make a change to the itinerary while it is out for pricing? Or can I ask BA to request that D space is made available (since AY apparently thinks it should be)? Thanks for any tips...

I think so, don't know about D availability but aa.com definitely shows both flights.