There are butter-flavored cooking spray oils that will do the trick. I run a volunteer sports snack concession and that is how I get the salt to stick to heated pretzels.
I use a quick mist of Pam over the Popcorn then sprinkle the Spices. The Pam doesn't alter the popcorn flavor and it is like zero calories and edible....and most importantly your spice flavors stick on to your popcorn.
Depending on how much spice you want I usually mist the Pam on then sprinkle seasoning stir...then repeat to flavor level you like.
If you pop them in a bit of extra virgin olive oil, it should stick quite well. It's a bit more viscous then butter spray and it's fairly good for you as well.