A mediocre Event System would give you the fundamental features of Item Encumbrance.
Having a Variable that sets as the whole weight that you can carry, that changed with switches;
Then you can do conditional branches, if New Weight is heaver than Total Weight, the item wont
be taken, but still remains. If your New Weight is lighter than Total Weight, than add the weight
of the item to create the New Weight. However, you would need to create a Common Event for
every single item so that when it's used, the weight would lower.
The problem with that is that it doesn't apply to selling items, buying items, weapons, armor
and enemy drops. The only way this would work is if you gain items only through events.
But since this is a Script Request, I'm not needed here.