Re: Flying out of Narita , guitar in hand
Often, an electric guitar in a soft case can lie on top of other soft-sided luggage in the overhead bins. It's often shallow enough to fit. But I don't know what JAL procedures are. If they're really strictly enforcing the limits, they may not even let you try to fit it in a bin.
What I would do . . . since we're talking about a bolt-on neck, and assuming we're talking about a foldable "gig bag" style case, I would first separate the neck and body. Then wrap the body with clothing like t-shirts, scarves, boxer shorts, etc., and place in case. Then wrap the neck with socks, tees, shorts, and slide it in on top of the padded body. Fold the neck portion of the gig bag over the top of the neck and secure with a few wraps of masking tape.
Good luck! Oh, and what kind of RG? Post some photos and give us a shopping report!