I started hacking on sccache for my quarterly deliverables a while ago then decided (also a while ago) to rewrite it in Rust, since that should help manage some of the complexity. It's going pretty well, I'm almost at feature parity with the original. My issue for tracking feature parity is here: https://github.com/luser/sccache2
This is basically done. There's one item left on that feature parity list (terminate the sccache server after 600s of inactivity), but it's not actually blocking using it in production because we have a postflight makefile that explicitly terminates the sccache server there.
I'm going to call this fixed. bug 1286934 tracks enabling it in automation, and it's very close.