How to build unlocked flakes on hydra?

I’ve set up a build server with hydra and I want it to rebuild my hosts’ configuration as nixpkgs master advances.
My hosts’ configuration is a flake file with hydraJobs containing a derivation per host.

First thing I’ve noticed is that there’s no way to do an --override-input in Hydra. No worries, I can create a directory of subflakes that do exactly that and build them instead.

Second thing I get is cannot write lock file. OK, patched hydra to add --no-write-lock-file.

Next obstacle is

hydra-eval-jobs returned exit code 1:
error: cannot update flake input 'NixOS-nixpkgs-staging' in pure mode
error: worker error: error: cannot update flake input 'NixOS-nixpkgs-staging' in pure mode

and the code around it suggesting hydra is really bent on building locked flakes only.

Is what I’m trying to achieve going against the flake philosophy or what? In general, how am I supposed to autorebuild flakes when their inputs advance?

Suggestions of any kind are welcome, because the best I can think of now is nix flake update && git commit flake.lock in a cron job.