there is no built-in Tableau system id that is out of the box, however for sure you can provision an id to be leveraged by tabcmd. This is what we have done, we are integrated with Active Directory so we had our security team create an app id (e.g. xxx_Tableau_SVC) that we then imported into Tableau. And then the id became accessible by tabcmd for being able to write scripts that are not tied to a specific personal account.
That makes sense, when we use an active directory userid it works, but none of the system id's work, but none of them were in active directory. Thanks!