lbschenkel
This tweet https://twitter.com/GrapheneOS/status/1671754760364478467 is somewhat relevant:
The Pixel Tablet running GrapheneOS works with the Charging Speaker Dock as both a charger and a speaker. Smart display features in the stock OS that are activated by docking require privileged Play services integration and likely won't be available via sandboxed Google Play.
I am not sure whether the hub mode you refer to is the same as the smart display features.
Casting does work on GrapheneOS but it's usually up to the app casting to implement it.
In any case, hopefully a user of the Pixel Tablet will be able to provide you with more information! There are quite a few Pixel Tablet users in the GrapheneOS chatrooms on Matrix.