Bedlining it would be tough to keep it clean, Plastidip would peel off easily, vinyl is a decent way to go but you will end up replacing it every few years. The best bet is Krylon Fusion, made for plastics and pretty tough. Also there is bumper coating, sold at your local auto parts store, also made for plastics, but this doesnt have a glossy shine to it. Post up some pics after youre done, we would all like to see it!
BTW Proper preparation prevents poor performance! If you have questions on preparing the surface just ask away.