Velcro? But i would say that would fall out from the weather....ummm i mean that is a really hard solution....use some sort of glue or wire ties but with wire ties you need a part to mount it with which id say drill holes......
Some people i've heard have adapted license plate mounts so that they fit through the lower grille and outside to mount the plate on, but i've never really seen anyone successfully mount anything directly to the bumper without having to drill....are you required to put one in front according to state laws? Or are you just looking to add a novelty plate?
I saw some one take one of those backing plates from a car store and retrofit it so that the top of the lower grill lined up with the top of the plate. The plate hung down lower than the front bumper by just a bit. Didn't look too bad and there were no holes.
my sisters fiance was court ordered to add a front plate to his 350z, which is custom candy painted, and he didnt want to... so he made a bracket that went from the lower grill, and bent upwards to mount the plate to and bolt it inside the bumper to some metal in there... totally secure, and hands about 1/2 inch away from the bumper