I wonder, then, whether annotation is also all they do when they mark that a child is flying with an adult on a separate PNR (which I have done many times on AA domestic, and everything, including upgrades works). If that is the case, the Iberia agent could write in the record "Accompanied by parent who is on reservation AA PNR xyzxyz"

I am now recalling that this past summer I did take a flight with my daughter: DFW-LHR-CPH and then FRA-DFW, which I paid for with $ for me and then the agent booked three miles segments separately for my daughter and linked each to my ticket (I recall asking about irop at LHR, with the agent responding not to worry). The only difference here then is that the Iberia agent will either have to be able to see the AA reservation, or simply trust me to provide that information <grin>. If that works, then there are seats, but I will cross that bridge later.