The NAS is a GGSN and it allocates IP address dynamically off the pool.
SBR is not doing Authentication, neither is the concern. Only Accounting message is sent to SBR (Proxy) so that it can forward it to appropriate End-node (D1/D2) based on Accounting Attribute Maping.
You might be able to get the GGSN to send an attribute with the Accounting request that you can use, but I don't know what the GGSN can do.
It would be great if you would post a typical Script for selecting realm based on framed-ip-address segment.