I can see the merits of this strategy and how it will seriously hamper japanese expansion. I also appreciate that an unsuspecting japanese player will have to make some difficult decisions as a result of this strategy. That alone is a very powerful effect.

What remains to be seen is if japan would be dead g6 as you said. If the japanese player ‘turtles’ as germany usually does it would become costly and difficult to invade them. If for some reason japan is only contained and not destroyed you’ll have a very large and dangerous germany to worry about.

I like your idea, its novel, but I’m not convinced that this would be as simple as advertised. The key, as stated already, is to create options for the allies to go either way. If Germany is entirely ignored you lose said options.

Nonetheless its an interesting idea.