Just extend the MainPage class.

That is, after the models load but before the setup runs, add something like:
```javascript
(function(L) {
  var OldMainPage = L.views.MainPage;
  L.views.MainPage = L.views.MainPage.extend({
    render: function() {
        OldMainPage.prototype.render.call(this);
        this.$el.prepend(L.templates['warn-persistence']({loginState: L.server.get('registered')}));
        return this;
    },
  });
});
```
see `src/js/platforms/firefox/models/preferences.js` for an example. (Although don't do what I did there, I was lazy and that really should use proper inheritance. Feel free to fix it :grinning:)