Make a QR also act like a Trigger Link. Whatever the Contact sends via the QR is included in the Contact record which then allows a Workflow to follow up (send an email back with the promised lead magnet!) and assign to a user.

If a QR code is scanned an automation ie. such as inputs a keyword into an SMS messenger. Many QR code generators can do this today. Then the person can on their mobile device press send to opt-in to a particular campaign.