Can a person use a metal handybox cover with a 1/2 knock out to run MC cable out the front of a plastic box?
Yes, either the cover or the extension ring would be acceptable.
Both the cover and the cable sheath need to be appropriately grounded. If using the extension ring you can use a simple grounding clip and pigtail it to the circuit's equipment grounding conductor. The clip won't fit the beveled edge of a handy box cover so you would instead need to use a bonding bushing on the MC fitting. 1/2" bonding bushings won't be at the home improvement stores so you'll need to order it over the Internet or visit a real electrical supply house.