Add target="_blank" to Alternative Registration Links on the Events Table template.

event_list_table_alt_links.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function ee_add_target_blank_event_list_table_alt_links() {
    wp_add_inline_script( 
        'espresso_events_table_template',
        'jQuery( document ).ready(function($) {
            $(".a_register_link").each(function() {
                var a = new RegExp("/" + window.location.host + "/");
                if (!a.test(this.href)) {
                    $(this).attr("target","_blank");
                }
            });
        });'
    );
}
add_action( 'wp_enqueue_scripts', 'ee_add_target_blank_event_list_table_alt_links', 11 );

instructions.md

Usage

You can add the above to a functions plugin or into your WordPress theme's functions.php file.