Dashboard widget custom positioning?

The dashboard widget function wp_add_dashboard_widget() is just a wrapper for add_meta_box(). So you can use the underlying function instead.

add_meta_box(
     'my_comments_user_dashboard_widget'
    ,'My comments'
    ,'dashboard_user_comments_widget_function'
    ,$screen // Take a look at the output of `get_current_screen()` on your dashboard page
    ,'normal' // Valid: 'side', 'advanced'
    ,$priority // Valid: 'default', 'high', 'low'
);