This is already possible today with existing caching proxies. This is a great way to make your CI/builds more reliable and quicker.

I meant a caching proxy that functioned as a mini Crates.io in the absence of actual crates.io being up. Depending on the crates.io protocol, just caching HTTP requests might not be enough, but also acting as a (offline-able) middle man that knows the protocol gives rise to other knobs and such (e.g. a configurable blacklist of packages).