A work in progress related to a WordPress stack exchange question.
EXAMPLE
For an example, take a look at the example_plugin folder and the register_addt_plugins_dir.php file.
You need to copy this folder to your actual plugins folder and then activate it from within your plugins list. It only works if the main plugin is activated.
*) the one you defined with the WP_PLUGIN_DIR or WPMU_PLUGIN_DIR in your wp-config.php file - or the default plugins folder in your install.