@Andrew
We are able to complete the end-to-end ONLYOffice Integration with GLUU in our test environment, it turned out to be ONLYOffice setup issue , not really GLUU Side :)
As discussed briefly today the kind of changes ONLYOffice setup is misisng, in continuation to that context, we can help you further over a screen share session tommorow towards setup on your side, and we should be good then.