FreeRadius-supported Billing System?

[Nick]

Hi all. I'm developing a VPN service (with FreeRadius and MySQL as
backend) and wanted to know if there's any customer/billing management
solutions that support FreeRadius before I go and write my own. I
haven't really been able to find anything other than maybe Platypus and
a plugin for WHMCS...

What solutions are out there that might be appropriate? (commercial is
ok). Could I maybe adapt some of the more WISP-oriented solutions to my
needs?

Thanks in advance,,
Nick