>Does a no-foil brisket need a rest period?
IMO, briskets don't require a rest period. The reason most folks do rest in the cooler is a matter of timing the meal. Because they take so long and are unpredictable, you typically aim for a finishing time several hours before meal time, just in case it takes long. When they finish early, you need to keep it hot, hence putting it in the cooler. If you're serving right out of the smoker, you slice right away, or maybe rest in the open for 15 minutes.
>won't the rest in the cooler prior to slicing soften that bark I worked so hard to obtain?
Resting in the cooler with soften the bark, but IMO, that's not a bad thing - it's still bark. But if you want it to not soften at all, then you'll need to serve it up right off the smoker.