The above MIGHT stop it provided you get the very end of the crack. But you don't know where it is, only what you can see. The crack may be microscopically propagated all the way to the top of the head tube, just not visible to the naked eye.

While the clamp sounds like a good idea, the truth is you don't know how far the crack goes and the only way to find it is to magnflux /die and black light the head tube. It could just break over the clamp.

Maybe try three clamps and some duct tape.

If it was a strut on the wing of a plane would you fly in it? You're shooting craps with the devil on this deal.