kind of reminds me of etc in NixOS but expanded to any file.
Yes but also with impurity! 
With good reason, it’s outside of what you’ll find in nixos / nixpkgs, as it loses reproducibility guarantees, in exchange for iteration speed.
I would recommend also checking out mnw. It provides a devMode, which lets you apply Lua config immediately while in the devshell. The only time you have to actually build is when you add a new plugin/binary to the closure. I use it personally, and have had a great experience with it.
… and then today I realized that if I’m managing not only my personal systems with full nix config repo checkouts, but also servers and nucs and whatnot that only get nixos-rebuild --target-host deploys, I would prefer the full wrapper including -u {init.lua} (ending back at @fzakaria 's post)
So I added an initLuaFile option that does exactly that. Basically extending @viperML 's neovim wrapper from scratch to make that part optional.