Small update - I've found a workaround for getting the Unity Editor to properly authenticate brainCloud with the Meta Quest version of the app (however an actual PCVR Rift build still does not work). In the Meta dashboard I created a "test user" under the Quest version of my app, and in the Unity Editor's "Oculus Platform Settings" there is a checkbox to "Use Standalone Platform" which allows you to input test user credentials for the Meta Quest version of the game (see attached picture below). This apparently allows the Meta entitlement check/nonce to be associated with the Meta Quest app ID - only in the Unity Editor, not in an actual PCVR Rift build.
So, this appears to be a workaround to get brainCloud auth to working in the Unity Editor for the Quest version of the app by using a "test user". Note that this is not a fix for getting the Rift version of the app to work with brainCloud auth alongside the Quest version. It is simply a workaround for the Unity Editor only.