Due to the considerable disruption to US flights today, I would be reasonably confident that somehow you will get yourself to ORD, I suspect you will have no problem with getting routed there, once you get to the USA, and ideally get to an Admirals lounge somewhere (which unfortunately there isn't in IAD). I'd also not get too stressed by this, since if you try to make a complex arrangement in London, a lot could change by the time to get to America.

What I would do is not bother calling at this stage (but you best have the AA numbers to hand, they have a different approach) and get to LHR T2 as soon as you can, and see if they will simply re-route you to ORD. I would be pitch for the direct services to ORD from LHR if you can. Then try again in DUB and so on. If no dice, then as I say even at BOS, if horribly late the AA staff will simply re-route you to ORD on the next flight. I also wouldn't assume you're going to miss your original flight either. on today of all days.