You should not count on doing this outside of an involuntary reroute.

I have done it twice on UA when flying AUS-ORD-BWI, I asked for a standby to DCA. The first time, I got the boarding passes in AUS. The second time, AUS listed me on standby but checked my bags to DCA.

When I reached ORD I was given a boarding pass for DCA but only because my bags had been checked there....I noticed a notice on the podium that said that BWI tickets had to upgrade to full fare. So I didn't try again...

As I said before, you should be able to do anything you want during an involuntary reroute. I remember once my DTW-LGA on NW was cancelled and I asked for a reroute to Binghamton or Syracuse (closer to my actual destination...), and they gladly accommodated me. Maybe the fact LGA was sold out for the next day or so helped my case.