Will we maybe also get official musl-host builds accessible via rustup and automatically fetched on such a platform?

We are always interested in adding more platforms. I don't remember if there are any open issues with a MUSL host; I vaguely remember something about compiler plugins?

I suppose the issues you're referring to are with statically linked against musl executables. I wouldn't expect any such issue with musl just as the host, where you can run rustc on alpine and voidlinux, just not necessarily statically.

rustc itself loads stuff as a shared library, in my understanding.

Speaking of non-tier1, is there support for static linking libc when building on/for FreeBSD?

I am not sure, but I don't think so right now. I don't know how FreeBSD and MUSL interact.

I mean given that FreeBSD's libc allows full static linking, the same support as for musl-target.

I assume so, I was just trying to say that I am ignorant about BSDs in general, and don't know what the status of any of it is personally.