I have been known to over-engineer things, so when I use WCF in my web applications, I wrap the service in my web app. This way my web app calls the abstraction.
Now, what you can do is apply your code access security (CAS) on the wrapper.
Example code might look like this (tons of details omitted for brevity)
internal class ServiceWrapper
{
Service Svc;
public ServiceWrapper()
{
Svc = ServiceClient();
}
[System.Security.Permissions.PrincipalPermission(System.Security.Permissions.SecurityAction.Demand, Role = "HelloWorld")]
public string HelloWorld()
{
return Svc.HelloWorld();
}
}
In a perfect world, we would want CAS to be a bit more dry (don't repeat yourself), meaning handled in the WCF as you suggest. But this might be a good middle of the road if know you can lock down your WCF app and control who calls it :-)
That would help you simplify getting the ball rolling...
Good luck!