I've tested pinning using both a free and premium account without luck. 'Outlook on the web' is referenced in the documentation I've linked to above but only briefly. I can currently pin the task pane in the Outlook for Mac client but have not been able to find any information about pinning on Outlook on the web. I've implemented the SupportsPinning element in the projects manifest with version override support for schema versions 1.0 and 1.1 using the example provided at.
I'm currently working on a custom Outlook add-in and having trouble finding additional information on support for task pane pinning.