i bet if you scuff the surface first (say 2000 grit sandpaper) and spray almost anything on it, it'd stick. something like this it would be best to remove the grill and not paint while still attached. Its ok to paint on the jeep but if you want to make sure you cover the shiny beyond the shadow of a doubt i'd make sure to scuff it. plasti-dip seems to be poplar on here these days
what ever you do - just take your time and good luck!!!!