ONT. CircuitID isnt provided by chorus in any communications so an RSp cannot populate anything with the details. RemoteID on the other hand is simple enough as its the string CHORUS with the customers ASID appended. (for UFB).
This changes a lot less than the circuitID would, potentially.
So in an IPoE setup you can auth the connection based on the remoteID which is passed to you in DHCP Option82. Which as discussed is inserted by the ONT.
Mapping back to customers and making sense of accounting is an exercise for the reader:)