The way he did is,
1. Just hook into player and change gravity. It is much easier than changing entity gravity.
2. Register substitution mobs which has less gravity value,
and make Overworld Mobs can't spawn/live in the dimension. (like Moon with no air..)
So.. it would be kinda depressing;
I think your first way would be the best.
& To 61352151511 : patching them all would be impossibly hard, since there is too much hard-coded entities..
EDIT: Galacticraft seems to do the patches, too. (Using MicdoodleCore, maybe.)