Tracking Unsubscribe Email Source

Hi all - looking for some help/guidance!

We recently updated our preference center to include an unsubscribe reason. As we are analyzing the first few months of responses, we realized we would like to include a field with the name of the email that triggered their unsubscribe. Ideally we want to track trends to see what types of emails are resulting in unsubscribes. 

I've created a new custom string field to capture this data and all the preference center links in our emails have the ?mkt_unsubscribe=1&mkt_tok=##MKT_TOK## at the end of the URL... Now I'm a little stuck.

I'd like to use a triggers and tokens to streamline/automate this process going forward to populate the new email unsubscribed field. 

I have a hunch that I will have to add a smart campaign to every email send moving forward but I'm hoping there is a better way! 

In a perfect world I'd also like to backfill that field with data for people who have unsubscribed in the last 6 months. 

Does anyone else track the Unsubscribe from email activity? If so, can you share how you've set it up?

Thanks a million