I can override the tree growth using the features modifications of a datapack or overriding with kubejs, but this only works for trees spawning into the game via worldgen (or using the place command)

There doesn't seem to be a way to do this for sapling growth, as these generate the bigboy trees regardless of how the feature says it should grow.

Am I overlooking something, or is this hardcoded and would need to be overridden with another mod addon?

How does kubejs let you override sapling growth, can you provide an example of the file or docs? We do force the tree growth differently but it should still use the same placement ID as the worldgen ones.