"Literacy and Script Reform in Occupation Japan" is at best not idiomatic English. What else might matter?
Whether or not it's a published title, what you Posted isn't incapable of being understood but "(anything) in Occupation Japan" at best fails to be a synonym for "Occupied Japan" or "Japan during the occupation."
Is it not obvious that "Literacy and Script Reform (anywhere or when)" will refer to something more obscure rather than anything generally understood?
How could (for you) the rare use of any expression matter? Would you rather drop that or show both where and how you came across it, and then how it might matter?
Is it not clear that there being a "post-occupation Japan" is at best a flimsy hook on which to hang there being an "occupation Japan", right?
"Occupation Japan" is not idiomatic; less so if compared to "occupation of Japan".
Either way, "occupied Japan" or "occupation-era Japan" are not comparable; neither is "occupation Germany"?
Were those OK, they might have fallen into disuse and was that what you meant, or exactly what?