I have flown AS on AA issued codeshare tickets, and have been able to get an upgrade if it was available by inquiring with the gate agent. I have to get my AS boarding pass at the gate anyway. Just make sure the AS MVP number is entered for the AA reservation. Haven't had much luck by calling ahead.

A while back, I flew AA XXX-ORD, connecting with AS 23 to SEA. I had about a 2 hour connection, and after walking from the AA H gate to the AS L2B gate, there is nobody at the gate. I guess that I'm a bit early, and the gate will be staffed later by somebody who is working at the ticket counter. So I go out through security to the counter to check in, assuming that doing this will increase the chances of the UG. Ask about UG, and am told that FC is full with a waitlist.

I go back through security to the AS gate to wait, and when a different AS agent shows up, I show him my baording pass & ask about the chance for the UG. Expected answer- not good. He did change my seat to a now open aisle in row 7 or 8.

When boarding starts, I decide to hang back since my assigned seat is easy to board later. Right after the boarding line clears, the agent asks if "Mr. Smith" is here, and he happily hands me the FC boarding pass with a comment about a no show in F.

Last week, when flying on a similar AA codeshare, I head to L2B with a tight connection, and inquire with the GA. Boarding is about 1/3 complete. No UG today, FC is "checked in full" (as opposed to "boarded full"). My m/o in this case is be polite, show the boarding pass so they notice your name & status, and when the answer is no, happily say thank you. At least I have an exit row to stretch out in and a cookie from Starbucks.

Onboard, just as the door closing announcement starts, the forward FA comes to my row and say "Mr. Smith, I think we have to change your seat. Your invited to first class." Another no show in FC.

My own guess is that in ORD, DCA & LAX, there is enough paid FC travel that sometimes the passengers actually are late and don't make the flight, or they have purchased refundable tickets and don't cancel early.