M.F.
You can pump after feedings to save up some milk for your outing. Most people tend to be fullest in the morning, so maybe pump after your first morning feeding every day and freeze it. Defrost it the afternoon before you go out (it's only good for 24hours after you defrost it). You can use it to make a couple of 4 ounce bottles, then leave formula just in case.
As far as pumping and dumping, one drink shouldn't do anyone any harm. If you drink more, you probably want to formula feed through the night or used your stored milk, then nurse in the morning.