I don't know about those particular flights on the day you are traveling, but in general flying same day stand-by into and out of DEN on CO is rarely a problem. I did it twice this past week on flights into and out of DEN-IAH-DEN. Also, another stand-by option for you to DCA from DEN is through either CLE or EWR, skiping IAH altogether (I've done them all). If your wish is to get back to DCA early, the connection through CLE gets you there at 3:30 or 4:00 PM, IIRC. And, as JeremyZ has commented the CO people in DEN are quite friendly and easy to work with. If you want to fly the particular routing you mention I am sure they will try and get it for you.