Anyone familiar with UMichigan's waitlists?

The Office of Admissions told me that they divide their waitlist into three tiers: top, middle, and bottom.

They suggested people on the wait list may want to submit "additional materials", such as an LOI, latest coursework grades, yet another letter of rec, but *not* something like a portfolio.

I have long since graduated so sending in last semester grades is not applicable.

Does anyone have any idea what has successfully worked in the past or even this year, i.e. what (if any) supplemental materials waitlisted applicants have sent in that ultimately leads to an offer of admission?

Thanks!