Here is my advice..
Bedliner is meant for applications in a pick-up bed and is not intended to be a cosmetic finish. That said, it will be damn near implossible to ever get it cleaned, will fade and turn gray with time. Bed liner for a exterior finish is a horrible idea, heck, bed liner for a jeep tub in my experience is a horrible idea. easiest thing to do to get a black grille is search ebay or craigslist and look for either an already painted grill or just any grill that you can paint different times to get the desired finish. Worse case sinerio you endup hating it and can put your original grill back on.