I have an early 1993 CMC and my engine cover brackets are metal, too.
Wonder if they had a bunch of complaints on the plastic ones.
I, too, use back semi-gloss (Rustoleum or Krylon) just about everywhere that isn't trimmed.
The exhaust pipes (since they show below the valence) get a coat of semi-gloss high temp black each Spring. It's difficult to wire brush the exhaust pipes so I use a sanding sheet made for drywall - it looks like a micro waffle with squares. Wrap it around the pipe, give it a few pulls back and forth and move on - works great.
Does anyone have a good quality paint that adhere's well to PVC? I want to paint the white PVC heater outlets on my car to sort-of match the carpet color. (Can you tell I'm running out of things to do????)