A simple plugin that will let you customize the default text that gets placed into a tweet. Works with Event Espresso and its social buttons add-on.

custom-twitter-button.php

<?php
/*
Plugin Name: Custom Twitter Button for Event Espresso
Plugin URI: http://github.com/
Description: This plugin will override the tweet button in Event Espresso
Version: 1.0
Author: Sarah
License: GPLv2
*/

if (!function_exists('espresso_twitter_button')) {

  function espresso_twitter_button($event_id) {
		global $espresso_twitter;

		//Build the URl to the page
		$registration_url = espresso_reg_url($event_id); 

		// custom default text
		$text = 'My custom default text';

		// new button
		if (is_ssl()) {
			$button = '<a href="https://twitter.com/share"';
		} else {
			$button = '<a href="http://twitter.com/share"';
		}
		$button .= 'class="twitter-share-button" data-url="' . $registration_url;
		$button .= '" data-text="' . $text;
		$button .= '" data-count="' . $espresso_twitter['espresso_twitter_count_box'];
		$button .= '" data-via="' . $espresso_twitter['espresso_twitter_username'];
		$button .= '" data-lang="' . $espresso_twitter['espresso_twitter_lang'];
		if (is_ssl()) {
			$button .= '">Tweet</a><script type="text/javascript" src="https://platform.twitter.com/widgets.js"></script>';
		} else {
			$button .= '">Tweet</a><script type="text/javascript" src="http://platform.twitter.com/widgets.js"></script>';
		}
		// all done!
		return $button;
	}

}