It should be just fine. Powdercoating is just paint, only instead of using a solvent (that eventually evaporates), or a catalyst (that reacts chemically with the base) it uses heat to melt the "powder", which then re-solidifies when the temp comes down. If anything, I think powdercoat would be the safest way to paint something in contact with your lips.
EDIT: you should run a few tests with nic juice before you actually USE it though. It's possible some of the chemicals in the juice (especially if there's alcohol to thin it, or carry flavors) could hurt the powdercoat and cause it to drip or... something else. This could also be different depending on which juice you're using. Think of all the plastic tanks that melt/crack with certain cinnamon/citrus juices, then think about the fact that powdercoat (paint really) is just another type of plastic...