Jessica, did you ever find a solution to this. We, too, provide our sales teams with some of our marketing emails so that they can send to their customers. This is completely separate from Marketo - no Sales Insight and we don't use SFDC (we use MS Dynamics). The URL approach that Jason suggested works fine, as long as the user has been cookied before and is known within Marketo. For example, they will see something like this:
But if they're not known, the email address will be blank and Marketo will not know who to unsubscribe. I wish there was a way to create some logic here - e.g., if KNOWN is false, send the user to an unsubscribe page that has a text field where their email address can be entered.