How to reference string keys in Flutter without context? 

In general, there is no clean solution here. 😔
In most cases, you'll end up with tightly coupled app layers.
Passing the context to the non-widget piece of the code (e.g. model class) that needs to be localized is perhaps the simplest approach.

class MyWidget extends StatelessWidget {
  const MyWidget({Key? key}): super(key: key);

  @override
  Widget build(BuildContext context) {

    var myList = MyList(context);

    /* ... */
  }
}

class MyList {
  final BuildContext context;

  MyList(this.context);

  /* ... */
}