One more data point: I've two successes in the past year checking bags with a long layover/stop.

1) IAD-SFO-SMF (early IAD-SFO with a meeting in town followed by a late SFO-SMF). Had to ask nicely but my checked bag was put through with apprx. 9 hours between my arrival and my brasilia onward.

2) DCA-ORD-MCI (didn't even have to ask when checking a bag through with a ~6 hour layover).

Also, for my money I'd take the 757 before the Airbus. 757 F is much bigger than the 319 and you'd be shocked at how easy it is to clear, plus 9C or even 9A on the 757 are my favorite coach seats in the UA fleet...far better than the 6s on the Airbii and better than the 12 windows on the 319 as well.