Originally Posted by ajwright

You may have to ask them to break the fare (i.e. split it into two flights). What you're describing doesn't sound like married segments, but a direct flight. So essentially you have 1 flight right now, that happens to stop in LAX. You have the extra bad luck of the two parts of the flight being operated on the same equipment - and for some reason, the system isn't letting you choose seats for the two parts independently.

I have had success with the medallion line before in getting them to split it up (in my case so I could get a C+ upgrade on the domestic portion). I might ask for help with the seats, and when they have trouble ask them "can it be done if you split the direct flight into two segments manually?" But you may have to HUCA. Just be nice, otherwise they may document the PNR and once that happens you'll have no luck.