The engine hadn't been run yet when I asked. It has now been run.
Before I started it, I ended up siphoning out as much of the liquid in the reservoir as I could, driving to a gas station, and topping it off with half coolant/half water.
Its a 2001 Mazda Protege.
The car didn't need a gallon of coolant, it needed wiper fluid. Had the right fluid, wrong hole. (That's what she said.)
Taking it to a shop wasn't really an option. We're a one car family and needed to get somewhere this afternoon.
As soon as I have the time/money I'll probably get it flushed and refilled. Until then, I don't think I'm too concerned. There can't be more than an oz or 2 of wiper fluid in the system.