It's not so much the workspace, but the partition and subscription.
Your lead is a member of many workspaces but there is only one email address inside your entire Marketo subscription. Even if you had many partitions, by default you'd have only one lead per subscription (the lead wouldn't be in multiple partitions, only one).
You could, in theory, allow purposeful duplicates so that you could have a partition for each workspace, and allow the lead to be a duplicate in each partition - this would solve your problem of unsubscription but would no doubt cause other conceptual headaches.
Given that this is a less than optimal solution, you will probably have to create some unsubscribe-lite fields that correspond to each workspace. Ie, when someone unsubscribes you don't update the lead to 'unsubscribed = true' (you ignore that), you update 'unsubscribed from workspace A = true". It's easier said than done to be honest but it's pretty doable. You'll need to remember to include these rules inside your smart campains when deploying emails.
Hi Laura Kimball,
Are the brands completely different and separate from each other? Could the unsubscribe page contain their preferences for all brands and ask them to unsubscribe from whichever mailings they no longer want to receive. So rather than hitting the unsubscribe button they would be checking the boxes for which subscription they don't want instead.