We have this:

options.systemd.services = lib.mkOption {
    type = with lib.types; attrsOf (submodule ({ config, ... }: {
      options.reloadTriggers = lib.mkOption {
        default = [];
        type = listOf unitOption;
        description = ''
          An arbitrary list of items such as derivations.  If any item
          in the list changes between reconfigurations, the service will
          be reloaded.  If anything but a reload trigger changes in the
          unit file, the unit will be restarted instead.
        '';
      };

      config = lib.mkIf (config.reloadTriggers != []) {
        unitConfig.X-Reload-Triggers = toString config.reloadTriggers;
      };
    }));
  };

In reply to @elvishjerricco:matrix.org
I... might just use that