How to declare nix flake registries when using nix-darwin alongside determinate nix?

[mitchmindtree]

I'm currently using nix-darwin to configure an MBP. Because I've installed nix with determinate nix, I've had to do:

nix.enable = false;

Unfortunately, this means any registries declared under nix.registry are also ignored.

This guide and the README describe how to add custom nix settings when using nix-darwin with determinate nix using determinate-nix.customSettings, however it appears this does not allow declaring flake registries as registries go in /etc/nix/registry.json (and not in /etc/nix/nix.custom.conf).