Proxy Forwarding on User-Name attribute

[Christian Meutes]

Hello,

i have the demand to forward some auth-requests to some further radius 
servers, but only in the case when a specific User-Name is for 
authorization requested. The User-Name attributes look like this 
"+493265352354 at realm.com" and the userpart is always changing but the realm 
is always the same. How can i accomplish this without defining every user 
which is to forward in "proxy.conf"? Is there no way to do this in the 
authorization backend(sql) for a more dynamic procedure?
I have thousands of users which are to forward and others which are not, i 
think this wont scale inside of proxy.conf and further more i think that 
there is no part of the User-Name which could be match in proxy.conf to 
proxy some user requests to different radius servers.
I would be happy if anybody could point me to an option to realize this.

regards,

-christian