Has anyone done it already? Not that I’m aware of.
Is it possible? Definitely, if you’re willing to do asm. This guide will get you started: GBAFE Assembly for Dummies, by Dummies
Changing growth rates might be a bit tricky since they’re stored in the ROM and not saved in RAM, but there are definitely workarounds for that. We already have custom growth getters in Modular Stat Screen, so you’d want to modify those to take the father into account. When you load the child units, you’d have insert custom code to check their parents and see who the father is, then save that somewhere (likely a free byte in the character struct). Then you could modify the growth getters to check who the father is - likely you’d have it reference a table, which lists growth bonuses/penalties based on the father.
At least that’s how I’d do it. Might be a bit challenging for a beginner to asm, but it sounds quite doable.
Edit: Original post was kind of incoherent, rewrote it to make more sense.