According to this source (https://dev.office.com/docs/add-ins/design/automatically-open-a-task-pane-with-a-document) at least an auto-open event exists: but only (1) for the plattform "Office for Mac. Build 15.34.17051500+" and (2) if the add-in is already installed on the client. That`s by far not enough und too complicated!
David Greenwood commented
It appears to me that saving an Excel file with task pane open does display on next open? Is this still an issue?