I assume you have a decent travel case/flight case for the guitar.

I'd trust JAL very far in terms of handling luggage that is labelled as fragile and packed in a proper manner.

I have zero experience bringing that large items in to the cabin. But with the current load factors I hardly see space as the issue. However, you'd need to notify them in advance, and you might have to pay a fee. Though I am pretty sure it can be accomodated. One way or another.
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