That functionality isn't available in NestedList right now, but you could get it to work doing an override like this:

Code:
Ext.define('Ext.overrides.nestedlist.TitleAlignment', {
    overrides: 'Ext.dataview.NestedList',

    updateToolbar: function(newToolbar, oldToolbar) {
        var me = this;
        if (newToolbar) {
            // create a new title component and add it to the toolbar
            me.titleComponent = newToolbar.add({
                xtype: 'title',
                title: me.getTitle()
            });

            if (!newToolbar.getParent()) {
                me.add(newToolbar);
            }
        }
        else if (oldToolbar) {
            oldToolbar.destroy();
        }
    },

    updateTitle: function(newTitle) {
        var me = this,
            toolbar = me.getToolbar();

        if (toolbar) {
            if (me.getUpdateTitleText()) {
                // Update the title component title instead of the toolbar title
                me.titleComponent.setTitle(newTitle);
            }
        }
    }
});
Notice in updateToolbar I'm adding a titleComponent into the new toolbar and save a reference to it. See I'm giving it an xtype just like you did above, and then give it the current title of the nested list.

Then in updateTitle, instead of updating the title of the toobar component, we update the title of the titleComponent we just created.

Ideally you shouldn't use this override though, you should extend NestedList for your custom view, and then override those methods in that class. But either way should work.

Hopefully this makes sense for you. Let me know if you have any issues.