But about using a Dremel; I've also always wondered if you can sand off thin layers on the surface to take off some amount of cambium to reduce the swelling which a lot of trees do around callus tissue.
The line is because of a bark inclusion - the two approaching lips had a thin bark covering when they finally came into contact. One might carefully carve this out and immediately cover with polyethylene, say, so the cambium doesn't desiccate (die back). The line may disappear much more rapidly that waiting for the natural merger of the two cambium lips that may take a very long time.
The 'using a Dremel' idea makes me pucker because it just makes me think of too coarse a resultant cut and doing more harm that good. One just needs to remove enough bark on the two sides of the lips so that the cambium on both sides can produce a common/continuous callus, somewhat like in grafting. Simpler things like just poking the line with a knife tip, an ice pick, or similar might be more effective, I'm not sure. Lightly hand sanding the line until a hint of green is seen and then covering might work as well.