Get a small tube of gilding powder (art shop or larger paint store) in bright brass color. Mix up some 5 minute epoxy and stir in some of the powder to make a brass putty.Quickly, trowel/force it into the gap with a Popsicle stick or plastic knife. Wipe off the excess. Wait until set but still sticky, and wipe clean with acetone. Then add just a little more powder to the joint and rub into the still slightly sticky epoxy. Let dry 24 hours, then buff by hand. It should make a good seal and a near invisible joint. I would try this on a piece of scrap first to make sure the epoxy is one that will set up hard.
Another that will hide the joint would be to fill it with JB Weld , wiping it with acetone to leave the repair slightly hollow, then use Rub-and-Buff (brass color) to hide the joint.
These two methods are how I often disguise the repairs on brass antiques.
Stacy