So I'm creating a mod which stores information about a player. This information is displayed both in a custom GUI element on their screen, and in their display name. I can change the display name using the NameFormat event when the player logs in, but it sometimes needs to change without the player relogging. Is there any way to either change the player's display name or manually trigger the NameFormat event?

I believe you can manually post events to the bus.  Try posting a NameFormat for that player.

You can indeed, thanks for that. One other question, regarding the EntityPlayer and String used when creating a new NameFormat event to post to the bus: should I use the player's display name or CommandSenderName for the string argument? The event is being fired, but the player's name is not changing. Could this be because the display name already has something appended to it?

The issue I have is that the NameFormat event only normally fires when a player logs in, but I need to update their username while they are still logged in.

That works perfectly, thanks!