Changing overhead name tag?

You'd have to change the player name, then somehow force their entity to respawn or they'd need to reconnect I believe.

 

You can use reflection to access/mutate "_displayName". "displayName" without the underscore is not a variable but a property that exposes the private variable _displayName. Because it's private, your only choice is to use reflection. A quick search on google should give you everything you need to do this.