/facepalm/
I forgot to preface my method with "@SubscribeEvent". No wonder I couldn't get it to do anything.
Okay, yes, removing the item from the list prevents it from dropping, but it doesn't keep it in the player's inventory. The item is just deleted.
I tried adding the item back into the inventory after removing it from the drops list, and now I can see it in the inventory hotbar when the player is dead and the respawn menu is up while all the other items have been dropped to the ground, but once I respawn, the item is gone again. And it still didn't drop.