Override Player Display Name

Hello, as the title says I want to override the display name of the player. 

In my mod I have a status effect called "Anonymous" that hides the player's nametag and changes the player's skin, effectively making it so you can't tell who that person is. These features work fine. 

What I want to do is make it so whenever the player would do something that appears in chat, like kill someone for example, instead of saying "XYZ was slain by ABC" it would say "XYZ was slain by someone". I tried using the "Try to set custom display name" block to do this but it doesn't work. Does anyone know how I might achieve this?