How to invoke mass action externally

Hi,

It would require a new system plugin implementing the trigger onMassactionTableTriggersLoad(&$table, &$triggers, &$triggers_html, &$loadedData) (to add a new trigger for your link URL) like we do in the plugins/system/hikashopmassaction plugin with the cron task triggers.
Then in your plugin add a second trigger onAfterRoute and look at $_REQUEST to identify the parameters of your specific URL and call the massactionClass->_trigger function like in the onHikashopCronTrigger of the system hikashopmassaction plugin.