With skin on, rubs won't do much for it so you'd be limited on pretreatments to injections, providing you're getting through the danger-zone temp in <4hrs. That said, do you use, or have you considered, finishing sauces for enhancing the flavor? Something for dipping, or to brush on as it's served?
@Chef Jimmy J turned me onto using vinegar as a base (used in moderation it makes many meats that much more mouth-watering). I had used it a bit prior to this, but he is correct in his findings.
This is the only one I recall using that I formulated for beef, but I will suggest modifications for use with pork...this is NOT a typical BBQ flavor profile, by any means:
***
CHERRY BALSAMIC FINISHING SAUCE FOR BEEF
1/4 Cup ground tart cherries
3/8 Cup Balsamic Vinegar (aged 15-yrs) [may sub with reduce balsamic]
2 Cups water
3 Tbls Red Bell Pepper, freshly ground from dried, chopped
1 Tbsp black peppercorn, freshly ground
1 Tbsp Garlic, freshly ground from dried minced
2 Tbsp Onion powder
2 tsp Thyme, freshly ground
1.5 tsp Rosemary, freshly ground
1 tsp Oregano, freshly ground
1 tsp smoked Paprika
1/4 tsp Cinnamon, ground
2 tsp salt
Mix all ingredients together in a saucepan, cover and heat on low. Allow to slowly heat for at least 1 hour, stirring occasionally. If simmering occurs, reduce heat slightly.
Drizzle hot finishing sauce over freshly pulled beef and toss to finish.
***
Obviously you will need much more volume for a whole hog, as the above will be about enough for 6-8lbs. I would suggest adding apple of some form or another...1/4 cup of dried/ground apple + and extra 1/4 cup water, or 1/2 cup of apple sauce/puree...that will offer a smoother, more balanced flavor for the milder pork. You may also want to cut back the tart cherries by about 1/2 (reduce the water by a few ounces, if so), but I have found that when used in a dry rub, tart cherries with red bells is a very good match for pork shoulder and ribs.
For a really unique (in a good way) dining experience, add dried/ground blueberries (+ water), or fresh/pureed blueberries, to the finishing sauce...it's is fantastic with much of the above ingredients when used for a pork rub, and could only serve to make the finishing sauce that much better, as well.
http://www.smokingmeatforums.com/a/blueberry-cherry-dry-rub-for-pork
Good lookin' pig roast going on in that photo...
Eric