Copper can be turned black by using black patina from a stained glass store, if it's just one piece I would take it in and ask them if they would quickly do that for you, it takes just a few seconds, or I've heard of using stove blackener, it's used to spruce up wood stoves, or there are cloths you can buy that have blackner on them to spruce up your stove elements. I'd even try good ole' black paint, maybe on a decoative wood overlay?!

Coco, I don't suppose it's at a point on the post you could put big square house numbers, like tile numbers, or brass? You probably would've thought of that.
Is it just one or two horizontal shallow slashes... or deep cuts?

Hmmm, if the scar is in a good spot for a small plaque you could have one made that says "close the gate, please". I have used a custom engraver on eBay that was not expensive at all. I think his shop is called masterblaster.

If the scar is low, how about a copper cat, just tack it on with tiny copper nails. You can sketch a pattern on paper, put it onto copper sheet with carbon paper (or eyeball it!), and cut it out with tin snips, or if you get the real thin copper just old scissors. After it is up... spray it once in awhile with straight vinegar and it will turn mottled green.

Some boo-boos I like. They are fodder for creative juices.
GH