On flights that are kinda empty and not running late, I have had no problems asking the pilot to hang out in the cockpit before takeoff before. The flight once was held for about 20 people with late connections (last flight of the day) and I started speaking to the F/As while the cockpit door was open and the pilot joined our conversation of Snickers vs. Milky Way. The pilot agreed with me (Milky Way) and then invited me and another friend who were up there to come into the cockpit. We talked for about 10 minutes before he kicked us out. We joked around a bit with him and he told us about his son going to college soon (I'm in college). All in all, a great experience. He even let me sit in the captain's chair for a minute. Good times, good memories, and a good way to keep me going back to USAir.