This is a common desire. IIUC hydra technically has some capability here, but it’s disabled on hydra.nixos.org because it sends too many notifications.
Here’s an earlier thread: When hydra fails to build a package, does it notify maintainers?
The thread above has an example of a tool someone’s using to regularly build things they depend on in CI.
It might be possible to build something on top of the --dry-run flag to help identify paths you expect to fetch that want to download?