Take a look at extending the security model. There is sample code on MSDN.
http://msdn.microsoft.com/en-us/library/aa237550(v=sql.80).aspx
In particular:
IAuthenticationExtension Represents an authentication extension that can be used to authenticate users in Reporting Services.
and
IAuthorizationExtension Represents an authorization extension that can be used to provide a way to authorize users in Reporting Services.
🙂
We are the pilgrims, master.
We shall go always, a little further.