It could be stabilized with a clamp, but what kind of clamp is the question.

I repaired an "identical" frame where the seat lug had the same sort of crack going out towards the top tube. I used an external-type, cast-aluminum seatpost clamp, "mangled" in place to fit well and clamp tightly. The frame went on to provide years of riding.

Just over two years ago, I made a very heavy steel clamp to support a bracket that spared a bb-shell brazed joint from crack expansion, has survived many CX races since then with no further issue other than the significant weight of this repair. Your repair would seem to be much simpler to execute, depending on the materials at hand, your imagination and engineering acumen.