Seems pretty easy to me.
Banzai charge could trigger on either or these two conditions;
1) Attacking: Japanese reinforcements are reduced to 0 and all Japanese players are dead. All Japanese players immediatly spawn with their last selected weapon with no ammunition. This forces players to rush in banzai form by;
a) giving them no ammunition to camp
b) no more reinforcements means that the team will need to attack en mass as a group in order to take the final objective. If they dont mass attack then they will get picked off.
This will help in making up for the difference in quality between basic rifles and possibly encourage people to use bayonets wherever possible
2) Defending: The final objective has an equal number of attackers as defenders or more attackers then defenders. Immediate forced spawn closer to objective then normal (ie will make it to the objective in time) with no ammunition and the weapon last used. Once per game only.
As both show, the easiest way to encourage players to act in a banzai charge manner is to remove ammunition which in many cases was the cause of a banzai charge. When things look bleak, time to banzai.
I'm more of a fan of the first example as it will not happen all the time, but will happen frequently enough to still give the side some character. I feel the second example may be too frequent