I'll answer my own question here. As of now, there is no way to add this link to the sales email signature (according to Eloqua Support).
The best solution I found at this point is pretty old-school:
1. create a mailto link and include your eloqua admin's email address in the "to" field.
2. Include pre-populated copy in the email body. Example: "please unsubscribe me from your email list".
3. Give this link to your sales team to include in their signatures (and/or add it to the signature rules in Eloqua).
That's it. It's an entirely manual process, but at least you won't get hit with a $1M fine!