I have a mixtape titled “The Optimist” and it is credited as “GZA & Friends vs. Tom Waits”. You can find it here on ebay: https://www.ebay.com/itm/The-Optimist-GZA-Friends-Vs-Tom-Waits-mixtape-CD-/264608465037
The problem is I can’t verify that GZA is actually the artist of this mixtape. I’m not sure how to handle that but also I’m not sure what MusicBrainz’s policy on mixtapes is in general. It seems notable and worth databasing to me as it shows up on a few blogs and Tom Waits rarities lists.