Two thoughts.
First, I'm not sure more US service into ORD and DEN is all that vital.
It's true that the codeshare requires a double connect to reach the western cities if you want to spend any of your time on US metal, but there's no reason you couldn't route through a single connection on UA metal (DCA-ORD-PDX or DCA-ORD-GEG in the examples you cited).
If anything, adding US service to hubs like ORD or DEN could dilute the alliance since (someone please correct me if I'm wrong), the DOT prohibited codesharing on routes where US and UA compete non-stop like PHL-LAX.
Take LGA-ORD, for instance. If US was to throw five or so flights on that route, it would no longer be able to place its code on UA's near-hourly service between the two cities. The result would be US passengers staying on US metal for longer, but at the same time, LGA passengers would certainly have fewer US-coded flights to choose among and probably lose access to codeshare destinations though ORD.
The only cases in which adding DEN or ORD service makes sense would be in cities that have US service but not UA service. Problem is, many of those cities are too small to warrant service to the UA hub (like SRQ-DEN, for instance).
Second, as for Bronner's comments on US being interested in ORD and IAD if UA is forced to fragment, I'm pretty doubtful.
IAD would be a nice gateway for transatlantic and transcontinental service, but when you combine the preference for DCA and National’s lack of low-fare carriers (though AirTran will come in soon), it makes more sense to keep DCA as your core operation in the Washington area. Plus, the last thing that US needs is another hub smack between PHL and CLT with DCA about 25 miles away.
ORD is a hell of a market, but US can’t just take a handful of gates here and there. In order for US -- or anyone else, for that matter -- to successfully compete with AA, they’d have to make a major investment in gates, aircraft, staff and affiliates. I just don’t see US being able to make that kind of commitment and I don’t think a partial commitment to Chicago would be profitable.
My guess? If US is eyeing anything UA owns, it would probably be some of the western ops (read: DEN). I think Bronner's comments about IAD and ORD are either red herrings to throw off the competition or an unprepared knee-jerk response to a reporter’s question.