Add "To" header without email being sent.

I've been trying to figure out recently for a custom messaging module how to add additional people to the "To" header without sending actually sending them an email. I didn't know if it was possible until I read in Wikipedia" the following:

Note that the "To:" field is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied separately to the transport protocol, SMTP, which may or may not originally have been extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter which is delivered according to the address on the outer envelope.

My question then is how to modify which emails are actually sent out. I've implemented hook_messaging. There I know the destination array only holds the person being sent the message. But if I add a "To" header, then that person is somewhere down the line included as a destination and sent additional emails. Is there another hook down the road where I can change the default behavior?