Meanwhile, I've made a system for items that modify a character's growths when held - much like FE5's Crusader Scrolls, both positive and negative growth modifiers can be used.
This hack uses one of the unknown bytes in item data, and makes use of an item's stat boost pointer; an item set to modify growths will use the first nine bytes of its stat boost as stat boosts as normal, and the following seven bytes will be read for growth mods, for 16 bytes total for the stat/growth boost array for a growth modifier (normal items only use the regular 9 bytes, as before)
There are two versions of this mod - one which allows multiple growth-modifying items to stack their effects, and one which does not. Both of them install in the same way.
Thanks to @Crazycolorz5 for his shortened level-up routine that I used as a base for this.