Thanks all for the thoughts.
My main concern in all this was getting the most storage life and quality out of the onions. I know a hard freeze softens the outer layers of the bulbs and make them unfit for storage (don't ask how I found this out). Really what I was trying to figure out was if a mild frost or freeze which doesn't cause any noticeable softening of the bulb still reduce storage quality.
Generally I have noticed that pulling onions before they start to wind down (tops falling over on their own) seems to lessen storage. Since I am in a short growing season area, I often have to decide whether to pull still growing onions before a freeze or chance that they make it thru unharmed.
I've decided to do an experiment along the lines of what Drew suggested (and hedge my bets on the weather some too), and leave some of the onions in the ground over this cold snap. I've pulled and am curing all the onions which started to show signs of winding down their growth (tops falling over). I've bend over the tops of the other (to give them a hint), and will cover them at night for some thermal protection. It's only forecast to go down to 29F or so, but that could easily end up at 22F too.
Guess I will see how this goes.