A lot of the above posts are missing the difference between coterminals for ticketing purposes and coterminals for standby purposes.

Unless things have recently changed, you may, under the rules, standby for a change from IAD to DCA and vice versa, unless the rules of your particular fare require travel into IAD or DCA specifically.

On the other hand, SFO-SJC-OAK are not standby coterminals, so the fact that you can't do it at those airports does not answer the OP's question.

That said, there is no IAH-DCA nonstop, so you would have to fly through another airport. Many agents won't let you do that on standby, so you'll need to get lucky if you want to try.