For many years, I just used an MSR 2-liter Dromedary or Dromlite bag as my pillow. If it had water in it, it had a bit of a waterbed bounce to it, but it worked fine and I never had a leak. If it didn't have water in it, I would use the smallest valve and blow it up like a football - worked as well or better than any of the air pillows they sell today. (Nowadays, I keep my raingear in my pack's lid compartment. The compartment clips to my pack, so it's easy to clip it off at night, use that as the pillow, and clip it back on in the morning. That assumes I don't have a down or pile jacket to use.)

It will work, but you are taking a calculated risk. Try it on an overnight trip in pleasant warm weather (where failure isn't critical) and see what happens.