Hi Jay,
Sorry for the late reply, I was very busy.
When a purchase order e-mail is sent, the backend uses the DocumentOutputRequest BO. This BO is released for partner development and you could implement a customer specific add-on that manipulates that e-mail. I know this, because I implemented an add-on a few weeks ago, that changes e-mail subject and text and the form to be used.
What did not do, is to add e-mail recipients, but when I had a look into that BO, I could see some functionality for adding additional e-mail recipients. Yet I am not exactly sure if this really works, so you might consider implementing a prototype first - I am quite confident that your tasks can be completed that way.
Hope to help,
Uwe