No Longhorn Server is its own OS. Just like Vista. It is the Server part of the Longhorn project. Just like SErver 2003.
Rang, no offense but i am one of the biggest users of Longhorn Server. I have used almost all of its betas. If i havent been able to find it as of yet it wont be able to be figured out here. You have to contact M$.
So my suggestion is to contact them. Cause not many others on this site have even heard of Longhorn Server let alone tried it. I triple boot with XP Pro, Vista RTM, and Longhorn Server Build 5744.
Like i previously said as a BETA it has many of it featured turned off. So it most likely CANT be used as a Radius server right now.