From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

Spotless is used by many of our third-party example builds: Gatling, Mockito, and now Arrow-Kt. We should have official support in Mill for a SpotlessModule to allow usage of Spotless in JavaModule, ScalaModule, and KotlinModule

Should be similar in style of PalantirFormatModule, ScalafmtModule, ErrorProneModule, etc.