Japan intervention is completely silly. Japan didn't had a similar ideology, it didn't had diplomatic connection and it lacked interest in the area. And I don't buy the argument of farming XP, because Japan population wouldn't buy it as well.
I believe that the ability to send volunteers far away from your borders should be highly restricted, needing a high political interest on the area.
Example:
One of the following three must be true:
- You have the same ideology (or quite close, like fascism and national-socialism. The international revolution is included here as well)
- You have a very high trade relationship
- Has the modifier "ex-colonial relationship" (one country had colonial possetions/policy inside the other)
The last one would allow US, UK, Germany, Japan and Portugal

to intervene in China. Other ex colonial regions can have the same system. Finally, although Russia bullied China as well, I would exclude URSS from the ex-colonialist list.