I would go the phone route as in my experience I can check in at least 1/2 before the official 48 hour time, which have not been able to do online. I have not pushed it any further than that though. I think its at the 72hr point that seats get released to check in? So maybe at that point if you get a sympathetic agent, they can then do it for you...

And also they have released seats for me not avaliable on the web.