I am in the same situation on a flight tomorrow. I booked a DFW-LAX flight which ended up being DFW-PHX on AA metal PHX-LAX on US air. I just called the Gold line to see if I could standby on a DIRECT AA DFW-LAX at the airport in the morning. She put me on hold and researched it and then came back and said since it was on both Carriers that the standby benefit did not apply.
I believe I am still going to get to DFW early and ask a gate agent if it is possible. My past experience with Codeshared flights even when the airlines were not merging was that the airline that issued the ticket was MORE than willing to yank you off a competitors metal to keep the revenue in the issuing airlines account. In this case since US is actually buying AA they may have squashed that type of thinking.
Any one that can actually say something about their experience with Stand by flights I would appreciate it. Especially if they originated at DFW.