To my knowledge, teachers can only assign mastery goals for the whole course, not for units. However, your idea seems quite helpful; I'll escalate it further so it can be made a future possibility. 

If you have any other questions, please let me know!

I think after assigning the mastery goal for the whole course, you can then assign an entire unit as an assignment.  I believe there is a guide in the help center that explains how.  I found it: https://khanacademy.zendesk.com/hc/en-us/articles/360007253831-How-do-I-use-self-paced-practice-and-Mastery-in-the-classroom-.  And here is a snapshot of their recommendation: