The wound looks as though it has already healed itself, but it is hard to tell. I learned in college that it is bad to put anything on it, so personally i wouldnt advise it. But it does look like it is healing. It looks as though it is grafted at the spot which it has cracked, that may make the graft weak, but it might pull through.

it does look well healed... ergo .. anything you do will be contrary to the trees ability to take care of itself .... relax... it took care of itself.. like most trees are able to do ...

it also look like it was grafted at that point... as the trunk below differs from the trunk above ... but i am no JM person .. so i might be corrected on that ...

good luck
ken

What I would do is to keep the winter sun from hitting the trunk during really cold spells - it isn't so much the freezing that does damage as it is a rapid thawing. Wrapping the trunk with a trunk guard, or just burlap, etc. should help protect it. I may be wrong and may be misremembering advice from a number of years ago, so am open to correction.

If you are still worried, you can post your photo over on the Maple Forum - it's mostly about Japanese maples over there.