Using a custom principal with COM+

Hello
I have a website (asp.net) that runs it´s middle-tier components (vb.net) in COM+, the site uses integrated security. I have the need to use a custom principal (need to attach some extra information on the user) in the web that i want to "use" in my middle-tier.
I have created a custom principal class that implements IPrincipal and are using the "Application_AuthenticateRequest" event in Global.asax to attach my custom principal to the thread:
HttpContext.Current.User = myPrincipal
Threading.Thread.CurrentPrincipal = myPrincipal
In my middle-tier i try to pick upp my principal like this (this is where my code fails):
Dim user As myPrincipal = CType(Threading.Thread.CurrentPrincipal, myPrincipal)
This work just fine as long a run the COM+ package as "Library" but does´nt work then I switch it to "Server".
Is this ...