Does such a thing exist? What good would it do to an NA engine?

I doubt it. For proper engine balancing, the crank has to be a certain weight. You must use steel (for strenth), and weight numbers between alloys and heat treats are all very close. You can lighten the crank somewhat, but the balancing effect must stay, so it's really difficult.

Well, that's true to a point, but our engines balance themselves. It's not like an inline or v engine that wants to jump or wobble. The flat engine configuration means that opposite pistons balance themselves out.
I belive B-spec carries lightened cranks...

Plus, oue cranks are so short anyway that they are inherently lighter than inline engines :)

Eric