Sadly, IFD is not supported in Hydra, since all evaluations runs on main Hydra machine (instead of slaves), it would slow down everything.
I’ve written about how IFD does help with development best practices as you’ve wanted to deduplicate expressions between the repositories to save developer time.
Until CI in nixpkgs allows IFD you’ll need to find a way to sync Nix expressions between your repository and nixpkgs.