Filter wp_mail based on content type

Note that the wp_mail filter runs before the wp_mail_content_type filter.

If you’re using the PHP Mailer then you can try (untested):

add_action( 'phpmailer_init', function( $phpmailer ) {
    if( 'text/plain' === $phpmailer->ContentType ) {
        $phpmailer->Body = 'Filtered text here'; // <-- Message override
    }
});