It don’t work that way for Afa’s Drops I’m afraid! They work by setting a bit in the character’s unit data (specifically the state bitfield at 0xC); during level-up, the game checks for that bit being set, and if so, adds 5% to every growth. You can only set that bit once.
Making held items or similar to modify growths is fine, but I don’t know if there are any free status bits to use for multiple Afa’s Drops.