In RADIUS messages, I'd like for the SM MAC address to be placed the Calling-Station-Id attribute from RFC3580:
For IEEE 802.1X Authenticators, this attribute is used to store the Supplicant MAC address in ASCII format (upper case only), with octet values separated by a "-". Example: "00-10-A4-23-19-C0".
In accounting requests, the SM MAC address is concatenated with a session ID in the Acct-Session-Id attribute. In authentication requests, it may be found in the User-Name attribute as the SM's default username, but there's no guarantee of that.
The Calling-Station-Id attribute would provide a key that can be shared between all functions of the RADIUS server and billing systems, and allow operators to set policies for an individual SM on the RADIUS server without having to know the SM username, password, or certificate serial number.
I should note that we're using exclusively PMP100s.