I'd like to manually assign the user for the current request from code.

For instance, consider an anonymous inbound request. If I caught that early in the lifecycle (like, BeginRequest via an HttpModule), I want to manually set the user as desired, like:

HttpContext.Current.User = GetDesiredUmbracoUser("foo");

I have done this in other systems, so I know it's possible from a ASP.NET standpoint, but I'm struggling with how to get the IPrincipal in Umbraco to assign to HttpContext.Current.User.

Again, the code above will work if I can just get a valid IPrincipal from Umbraco.