private void handlePower(String power, boolean isFromCo, ClientChatEvent event) {
  switch (power) {
  	case "immortality":
    	event.setCanceled(true);
    	if (isFromCo) {
    		Minecraft.getInstance().gui.getChat().addMessage(new TextComponent("[HellSchool] " + event.getPlayer() + " went immortal!"));
///                                                                                                    ^ not a method, so how do I get the player sending the message?
    	}
  }
}

@SubscribeEvent
private void onChatMessage(ClientChatEvent event) {
	boolean isFromCo = event.getOriginalMessage().contains("[CO-OWNER]");

	Pattern pattern = Pattern.compile("\\*uses (.*?)\\*");

	if (pattern.matcher(event.getOriginalMessage()).find()) {
		String power = pattern.matcher(event.getOriginalMessage()).replaceAll("$1").replace("*uses ", "").replace("*", "");

		this.handlePower(power, isFromCo, event);
	}
}

These are the methods I'm using to subscribe to the ClientChatEvent, the comments explain what's going on.

I need to get the player to identify the chat message's sender and add that message to the player's personal chat messages.

 

I think you can try this :

Minecraft.getMinecraft().player;

Darn, okay then. I thought the ClientChatEvent was universal and only hooked into messages in the InGameHud. Thanks anyway.