Not connecting. Originating in Seattle and going to OAK, but there is an earlier flight to SFO and that works too....I'm Gold BTW.
Then show up at the airport early if there's space and tell them you want to go to SFO.
If that's true, that blows the idea out of the water that AS can't allow routing changes when doing SDC because it requires a ticket reissue. If agents can do this, they should be able to do SDCs involving routing changes. It ends up being a similar process for the agents either way.
All I can say is you can't do co-terminal standby via SDC online, but I've done it at the airport loads of times.