Drushrc.php creation

provision_drupal_drush_exit() is called if there were no errors. It sets a bunch of options and then executes provision_save_site_data() which in turn calls the drush_save_config() hook which is what writes the drushrc.php, and then sets the permissions on the file.

Bit of grepping around will show you where the code is. :)

If you are trying to save data to the drushrc.php you should probably be doing that by adding an extension to Provision that does drush_set_option and things like that, to save the options to the running context before the drushrc.php is written