How to set defaults for a custom Views display?
The class views_plugin_display_page defines its options (and defaults) through the 
views_plugin_display_page::option_definition() method. This method is just a series overridden methods that extends from the view base class. In a similar manner, your custom plugin can override this method:

// defined in your views_plugin_display_page descendant class
function option_definition() {
  $options = parent::option_definition();

  // @TODO: set your own options plus override any other options
  // defined in parent classes. For example: 
  // $options['path'] = array('default' =&gt; 'my-default-view-path');

  return $options;
}