Go to the Battle Tower PC, select Lanette's PC, select Move Items, and give the Master Ball to a Pokémon.
Exit the PC and save.
Go back to the Move Items screen, take the Master Ball, and put it
back in the Bag.
Talk to the woman on the far-right, select a mode (I've tried both),
select the party, choose Yes.
Wait for the game to momentarily freeze and restart the game when the
screen changes (I've tried the soft reset and simply turning off the
handheld).
Pokémon with item:
Give the Master Ball to a Pokémon and deposit it to a box on Lanette's
PC in the Battle Tower.
Exit the PC and save.
Turn the PC back on and withdraw the Pokémon.
Talk to the woman on the far-right, select a mode, select a party,
choose Yes.
Wait for the game to momentarily freeze and restart the game when the
screen changes.
This is how to do it with a Pokemon with a item and just the item.
