As SJ said, I'm building a plugin for it outside the main repo to keep it separate from the Pants release process.
https://github.com/tgolsson/pants-cargo-porcelain
https://github.com/tgolsson/pants-example-rust
Does the basic building/packaging/linting, testing albeit a bit inefficiently due to Cargo being quite greedy when rebuilding. Not yet on PyPi so you'd have to depend on it from GH (per second repo), but my aim is to put it on PyPi ~soon. Happy to take PRs and feature requests :-)