One option would be to charge up an empty keg with CO2, and then use that to push the beer. The problem I foresee with this method though is that you'd be starting off with a fairly high serving pressure and then slowly decreasing as the night goes on. Maybe you'd get by with only 20 lb of pressure in the CO2 keg, but I really don't know how far the gas would go. Might be a good time to pick up a keg charger after all.