Add a .setMessage method to events, to allow setting the message sent (on success only maybe). This can be helpful for add-on's.

Examples:
(in this instance, TownClaimEvent)

e.setMessage(null) - removes message all together
e.setMessage("You were charged 5.0 for claiming this land!");

And an example with e.getMessage()

e.setMessage(e.getMessage() + " - you were charged 5.0.");

Nice idea, I'll have a look tonight

Edit:
Perhaps would be better as a collection, you could add to the collection or make it so its just your message is shown