Yes that’d be great. Regardless of the fact that this doesn’t actually help with the legalese, I’d love to have this feature when dealing with certain kinds of unfree software.
it would go counter to how the basic formats of Nix work. I have no idea how you forsee this to ever work with the already quite fragile remote builds.
I expect Nix would crumble, but it really shouldn’t: it has the recipe for provisioning the missing output by building at all times