What type of filtration does active carbon provide and does it make sense
in a pond?

My thought here was to place it in a bag before the intake of my filter.
This way I can miracle grow the VF, and not worry too much about the MG
making it to the main pond where the fishes are.

You might just want to use potassium, supposedly ponds (via fish food) supply
all the nutrients plants need except potassium.

The best potassium source I've found is potassium chloride at a water
softener store, $14 for 40 lbs, 100% pure (made to recharge potable water
softeners).

HTH