I am afraid I cannot. My RADIUS knowledge is poor. But I’m interested it this topic and will try to investigate.
In my test enviroment there are 2 RADIUS servers: ZS and another Fedora-based FreeRADIUS v. 2.0.5. I have added it as a proxy via ZS Web interface and it successfully authentificates auth requests from ZS. So for me it’s easier to tweak that second server. I’ll post my config when this problem will be solved.