This sort of crack is also in a spot where the frame sees a lot of tension in the main triangle. If it's a mountain bike that has seen a lot of jumping between the tension that results in G'ing out during a landing and the possible flex from the rider landing on the saddle at the same time repeated enough times could possibly produce such a crack.

In any event there isn't an epoxy (and JBWeld is JUST another epoxy) around that would do more than maybe keep out the water. None of them would make a tinker's damn worth of difference to the damage or propogation of the crack further down and around that weld.

If it were mine, and considering that I'm also a cheap bugger, I'd just sigh regretfully and saw it up and put it into the metal recycling bin.