Okay, so I went through this last year as a pair AA Exec Plat / Exec Plat, hence Emerald / Emerald; and am now doing it again Plat Pro / Plat, hence Sapphire / Sapphire. Everything works, but it takes a lot of steps.
- Go to Iberia.com. Buy ticket -- buy the cheapest one. Mine happens to be on Air Nostrum. I booked into Basic Economy. I did not join the Iberia loyalty program. I did put in our AA numbers. It appears to stick, and even shows up later, but really they don't, and it doesn't matter. By which I mean: Ignore the fact that it keeps trying to upsell you for seats and baggage, even though you have Oneworld status. Plow through at the lowest cost.
- After your ticket is issued and you have an Iberia PNR, go back to their site and find manage your reservation, or some kind of linguistic equivalent. Again, I am not being terribly precise here because some will be looking at these terms in Spanish, some in English ... mine happened to be in French, maybe because of ... well, who knows. Anyhow, once you are managing your reservation, there are a bunch of panels under reservation details. The first is passenger information. Click there. What you will probably see is that, in spite of what you tried under step 1, your non-Iberia (in our case AA) number is not there. But there is something to click on to put it in again. Do that. This time it will take.
- Once that is done, go further down the panels to the respective segments. Click on each in turn. You will see details of your flight, and then under some more clickable things, including (first two) seat reservations and baggage. When you click on seat reservations, look to the right, where it asks you "Why not choose your seat?" (or some linguistic equivalent). Hey, why not? Do it. It works like normal; sticks; and doesn't charge you anything, Clicking on the baggage panel will reassure you -- at least it reassured me -- that now and henceforth Iberia understands just who you are and plans to grant you checked baggage for free, in accordance with that status (I write as a miserable Sapphire).