There has been Grimrock's idea, but since it is a bit different, I decided to open it as a new Law Suggestion.
The issue we want to solve with this law, is to give nations a way to easily and effectively offer perks to its own citizens running plantations at their own ports.
The idea is to give a subsidy daily, based on the amount of produced units at the day that just passed.
The Law will specify a minimum and maximum value per 1000 units (hard limit of 1 and up to 5,000 gc).
Then, the Governor of each port will then be able to change this for his controlled port by choosing a value among the law limits.
Plantations owner will receive this, directly from the coffers right before the automatic daily payment for rent is issued.