Knockout punch for fiberglass?

Would one of those metal knockout punches be suitable for cutting holes in fiberglass? Or would I just break/crack the material?

You are going to smash a lot of the fiberglass around the hole. Make up a sample piece and see for your self. Not good unless the piece has a heavy resin presence which isn't usual.
I would never do it.
Route, cut, trim, sand, anything but smash with a hole punch.
Take a pair of vice grips to any fiberglass and you'll see the smashing effect.
My three cents worth Art

Thanks for the quick reply. Right after posting, I thought better of the idea and went and bought an appropriate sized hole saw instead. Not worth the risk of destroying the part.

Fiberglass composite fibers are actually glass, which means very hard. Consider this when selecting tools to work on fiberglass. In the case of the hole punch, it probably wouldn't cut a clean hole and the glass fibers would dull the tool. With a hole saw, same thing, the glass fibers would dull the teeth. I prefer abrasive tools to cut fiberglass. If I have to use a shearing or toothed tool, I mark this "for fiberglass only" once I have used it on fiberglass.