> eventually we'd like to use a first-class precompiled macro if such a thing becomes supported by cargo / crates.io

Distributing binaries is what distributions are for. It does not matter if it is for macros or for the rest of a crate.

Anyone can write and publish a cargo plugin that provides binaries via a separate channel.

https://crates.io/crates/cargo-prebuilt has been around for a while. (I've not used it, just looked for it now)