Not only does foiling soften the bark, water in the pan on bottom reduces bark formation right from the start, and if water is present throughout the hot-smoking it's not likely to get that nice, heavy, crisp bark you're longing for. There are some tricks for vertical smokers, regardless of their heat source. They don't perform the same way horizontal pits do. If you really want a pronounced bark, you need to know and practice a reliable method to create, and, lastly but just as importantly, preserve that bark. It starts with minimal water to aid in smoke adhesion on the meat, as humidity plays a role in this during hot-smoking. Then, you allow the humidity to drop by removing the water or only using enough to let the water pan evaporate dry about half-way through cooking. The reduced humidity allows the meat's surface to dry and tighten-up...this is where the really great bark I've produced has originated. Use only open-grate cooking (do not put meat in pans or on foil, etc) and never, NEVER foil...no matter what.
Plan ahead for longer cooking and resting times, and exercise patience with stalls. I like to rest pork shoulder at least 2-3 hours, but if you come up short on resting time, it won't kill your PP...allow a good hour, though. If it's done early enough for a 3-4 hour rest, you just won the bonus round. Rest on an elevated grate (bakers racks do well for this, or, your smoker grate) on a roasting or baking pan (to catch drippings and cut air circulation), covered only with cloth or paper towels. This still insulates well enough to keep it hot for a few hours and allows the meat to breathe as it rests, instead of steaming the bark from it's own evaporating water being trapped inside of foil. And, the bottom portion of the meat won't be literally soaking in it's own juices, so the bottom bark will be just like all the rest of it.
The key to preserving the bark is letting it breathe. If it's a heavy, crisp bark you crave, the results of this method will produce what you desire, consistently. And, no, the meat does not dry out, unless you over-cook it beyond recognition. Dry rub ingredients won't effect this method much at all...I don't use sugars, mustard, or any other pre-rub treatments. The bark is mainly formed in the surface of the meat...rub plays a role in the texture, too, but does not seem to be a major factor with this method (I've gone with naked meat and produced good bark). I rarely rub in advance...that happens immediately prior to smoking, and I don't temper the meat at room temp, either. That could be considered a dangerous practice with larger cuts of meat that are not intact whole muscle (see Food Safety Forum for more info). I apply my rub and go straight to the smoker.
Here's an example, with my favorite candidate for PP...the lowly picnic shoulder:
For more on the wet-to-dry smoke chamber method, see the Wiki article. I don't just use it for PP with picnics or butts...there's lots more I like with bark, but it's not just about creating and preserving bark. It's about the overall finished product's interior/natural moisture retention, as well:
I've been using that method for several years and it always gives predictable and repeatable results for moist PP with a killer bark. The best part is that it's easy to do...no special equipment or skills needed...I like easy.