The bikewrench community might have a fix. While I never saw a chainstay break like that before, I have seen breaks in load-critical joints such as downtube to bottom bracket shell and headtube to downtube.

My fix was to make an internal sleeve joint using carbon steel pipe. I would then epoxy it in place with 3M 5200, the same glue used to hold sailboat keels in place. Shipwrights also refer to it as Devil's Glue, because it's some tenacious stuff. The shaping on that tube complicates this fix, but some careful shaping might get you to done. The only reasons I even attempted these fixes were because the bikes had huge sentimental value to the owners.

Another possibility could be a internal shim made of G10, wrapped in ~5 to 8 layers of biaxial fabric (glass) wetted out with 2-part epoxy. Once cured, it would be ridiculously strong in all axes, for tensile, compression, and shear. Source: this is how I repaired a broken mast spreader bar in the middle of the ocean.

Effort-wise, you might be better off getting a new frame and using the original as an organ donor.

Also, YMMV, fix at your own risk, blah blah blah taco.

Edit: a word