wp_mail AddBcc filter

I need to do the same. Based on a quick glance through the wp_mail() function (http://core.trac.wordpress.org/browser/tags/3.3.1/wp-includes/pluggable.php) it seems that it might be possible to do via the headers section. I haven’t quite gotten my head around how to do it yet, but I think it might be possible via the headers argument.