I think you cannot access any custom information send on the REST call except the sender_id. Everything else seems to be neglected in the tracker object. I am also looking for a way to access custom information (e.g. user name and access token) through the tracker so I can make a REST call in a custom actions to e.g. store data extracted from the chat into a user profile.
Is there any easy way to fix this or must it be a change in the REST channel class and tracker class?
See a similar issue here: Customing user information in rasa