You might be able to do this using the 'AutoAuthenticate' hook found in StubObject.php.
Others may be able to suggest a better place.
-- Jim
I think this would occur too early for what he wants.
What we really need for situations like this is some type of stackable authentication system. I already have a planned way to stack auto-authentication plugins with regular authentication plugins (I just need to document it somewhere). I can probably create a plugin to stack regular authentication plugins while I'm at it...
V/r,
Ryan Lane