So I tried to write this plugin as:
add_filter('update_welcome_user_email', 'wpmu_do_apply_extra_filters_welcome_email');
function wpmu_do_apply_extra_filters_welcome_email( $content ){
return str_replace( "USER_EMAIL", '[replaced_email]', $content );
}
Seems like this should work since in wpmu-functions.php, the filter is applied as follow:
$welcome_email = get_site_option( 'welcome_user_email' );
$user = new WP_User($user_id);
$welcome_email = apply_filters( "update_welcome_user_email", $welcome_email, $user_id, $password, $meta);
$welcome_email = str_replace( "SITE_NAME", $current_site->site_name, $welcome_email );
$welcome_email = str_replace( "USERNAME", $user->user_login, $welcome_email );
$welcome_email = str_replace( "PASSWORD", $password, $welcome_email );
$welcome_email = str_replace( "LOGINLINK", wp_login_url(), $welcome_email );
I "could" make this work by adding a str_replace here, but much rather do it as a plugin if someone can help point out my mistake.
Thanks