The laws regarding radio frequencies are very restrictive in japan.

On the spec sheet for the DJI FPV it states that 5.8ghz is illegal in Japan and thus it is deactivated. However, this is not true. A pilot may use these frequencies if they have a 4th class ammeter radio operators licence or higher.  

Is it possible to unlock the frequency restrictions and strength settings on the DJI FPV?

DJI will not reply to these types of Questions (for legal reasons), you will have to go outside to get your answers.

I'm very sure there is a hack to enable 5.8ghz.    Google is your friend