We have run in to several cases lately where customers have FBA authentication enabled in the extended zone, but not the default zone, of the web application, and will receive errors when trying to use the Office Web Apps.
The issue stems from SharePoint and is due to an error in the access token generation when a user tries to load a document in the Office Web Apps.
This is a known issue, but there is currently no timeline on when or if this will be addressed.
The current workaround is to configure FBA authentication in the Default zone of the effected web application
First.Also, good article. Nice work.
I am facing a similiar issue on my ADFS extended Zone. Hope this is solved soon. OWA 2013 "expects" the default zone.