I don't inject my pork butts, but,(guess thats a double butt) the basic seasonins could be used, onion powder, garlic powder, cumin, cajun seasonin, greek seasonin, sumtin like good seasons italian dressin, fer liquids, apple juice, orange juice, or as others have said pineapple juice, a bit a whiskey won't hurt either. Ya could also melt a bit a butter an inject that too. Don't go to heavy, say a cup an half a water with bout 2 tablespoons a melted butter in.
I would start out easy on the injectin, work up ta a recipe ya like, it could get over powerin if ya use to much to soon. My feelins are, we are tryin ta inhance the flavor a the meat with some seasonin an the smoke, seasonin could over power a lighter smoke, thus the need ta experiment. Yall try some different ones an keep us posted on how it's a workin out fer ya. Should be a fun experiment as yall get ta eat the results! Good luck!