If you bulld the directed graph made by the symlinks in the nix store, and walk it backwards, a sha256 of the source files is what you'll find, both in the form of a nix store path and possibly in a derivation that relies on a remote resource but provides a hash of that resource so we can know it's unchanged when downloaded later.
The missing piece is that they're not gossipped between users. So if I find some code in a dark alley somewhere and it has a nix flake to make building it easy, I've got no way to take the hashes and determine who else has experience with the same code and can help me decide if it's trustworthy.
Hmm that feels a bit too much like a root of trust, those make me uncomfortable. I'm more interested in tooling for gathering metadata re: the trustworthiness of some code without the author's participation. If the author wants to be involved, all the better.
The root of trust has to lay at the source code origin for a pure implementation of reproducible builds and for the security reasons I mentioned earlier.
In general it doesn't help much IMO to have distributions take a silo view of the problem. But those are just my ideas and thoughts on the matter.