Most likely the ONU is NOT passing this data upstream to the DHCP server. By definition a Layer 2 CPE does not have any Layer 3 capabilities (well depending upon your system it may allow communication with the captive portal for entering the Registration ID
The issue is that your looking for the ONU to operate @ Layer2 as a bridge device.
One option would be to have the customer enter in the ONU as the user/pass into a captive Portal (hotspot) and then obtain an IP address based on what you had in Radius. This way if the customer changes their CPE the ONU is agnostic to this change and just used for the Serial Number Username from the customers view point.
Prior to DOCSIS being what it has become - we did this because of the 3COM CMTS being such a pain in the booty...
Not the best fix - but should work.
Glenn Kelley | MCTNA, MTCWE, MTCTCE, RHCE, RHCSS
USA Based 24x7x365 Mikrotik, Juniper, Ubiquiti TAC & WISP / ISP Blind Label Support Call Center