Okay after much testing I got this to work.
Im not 100% sure how efficient this is, but its kind of a really hard thing to work with.

You will need Skript-Mirror for this to work

Code (Skript):

1. import:
2.     org.bukkit.event.block.BlockPhysicsEvent
3.  
4. on right click on grass:
5.     if player's tool is any hoe:
6.         cancel event
7.         set event-block to moist farmland
8.  
9. on BlockPhysicsEvent:
10.     if event.getSourceBlock() = soil or moist farmland:
11.         set {_b} to event.getSourceBlock()
12.         set {_state} to {_b}.getBlockData()
13.         {_state}.setMoisture(7)
14.         {_b}.setBlockData({_state})