I have stood by for earlier flights when I couldn't SDC. There were times when I was able to add myself to the standby list before actually getting to the airport, but it has been awhile since I have seen that option. More recently, you can only standby by showing up at the gate.

Not sure how you standby for later - it seems to me one way would be to SDC as late as possible for an available SDC flight that was not the flight you wanted but after the flight you want to stand by for, then go to the airport and stand by for the earlier flight. Would love to hear any comments on how to stand by if you can't SDC to any later flight and you can't get to the airport until after your booked flight leaves.