To that end, I have this… silly thing, which may be helpful: Comparing NixOS:master...ElvishJerricco:run-in-fhs · NixOS/nixpkgs · GitHub
Basically, it takes a derivation and builds it in an FHS user env instead (i.e. you only need it if that derivation wouldn’t have successfully built normally). You still likely have to throw out… most of nixpkgs. But in theory you could build up a package ecosystem where all of them run in an FHS env like this and all of them rely on FHS stuff.