So I've not been able to make any progress on this. I've verified that I'm logged in with the correct account on both the Quest and the Oculus App; I've tried putting my credentials in as a test user in Oculus > Platform > Settings in the Unity editor, but they aren't accepted there so that didn't work (although I do get a different error with test user enabled).
Setting a breakpoint where the entitlement error is logged, I see no additional information on the error or the message it's delivered in. The OvrAvatarManager in the scene has logging set to debug, but there are no debug logs in the console. I can find no other way of getting more information on why this is failing.
I'm at a loss. Any suggestions / solutions would be very much appreciated.
