The (originally) bolded part is at the heart of the matter. If AA presented AA-based choices, but the UA-based choice was a better option, and the OP chose that one, it may be a tougher case. If that was the solution AA presented, then it may be easier. If it is the first case, I'd try AA again- just because it's easier. If it doesn't work, then the approach suggested above could yield results.
If the OP made the choice to go with UA, however, one might still try, but with appropriately-lowered expectations.
Cheers.
Thanks for the responses everyone.
There was no real choice in this matter due to the time. Taking the UA option got me home around midnight (just half an hour later than my original flight). If I took an AA route, the earliest I could get home was the next afternoon in which case I'd miss half a day of work.