Override Windows auth using global.asax?

Here's my environment
Win2003 Standard SP2, IIS 6.0, MOSS 2007, .NET 2.0.50727

IIS website uses MOSS 2007 "Team Site" template and has only IWA enabled.  Web.config has Windows auth w/ Impersonation:
    <authentication mode="Windows" />
    <identity impersonate="true" />
    
IE HTTP headers shows that NTLM authentication occurs for workstation user.  In global.asax, I've defined WindowsAuthentication_OnAuthenticate() and can see incoming the Principal/Identity values as:
    WindowsAuthenticationEventArgs.Identity.Name = ACME\testuser
    WindowsAuthenticationEventArgs.Identity.AuthenticationType = Negotiate
    HttpContext.User.Identity = null
    Thread.CurrentPrincipal.Identity = <blank>

I'm using the KerbS4U extension to cre ...