From what I've seen in source there seems to be no easy way of doing this unfortunately. However I'm not sure if sending all e-mails in BCC is really a good idea anyway. What I think might be the best way to resolve your issue is to actually generate a single subscriber for each of your "To" addresses. If you have more e-mail addresses in "TO" address, then why not parse this and create new subscriber for each?
If you could let me know in more detail where you store the e-mail addresses and how, I can look up a way how to generate new subscribers out of that.