I guess that the biggest challenge such a project would face is providing a smooth transition for package maintainers: if they have to cut-over or maintain two configs it will be much more difficult to retain them.

So my thought is an that maybe you should use a staged approach where your dhall packaging config design can be compiled down to nix, making it easy for existing nix packages to convert and still be backwards compatible with nix, then develop a dhall-only implementation that consumes the same config but eliminates the intermediate compile-to-nix and evaluate-nix steps. I don't know the design, so this approach may or may not be reasonable.

Regardless, I'm quite interested in a nix-like solution with configs written in Dhall.