Ok, well... Hmm.. I wonder if this would work.
The script requesting the file makes a window.location call to get the attachment then immediately makes an ajax call for confirmation.
This will bring into state 2 PHP scripts on the server. The one delivering the attachment would at conclusion right the session id into the APC Cache with a marker of "I'm done."
It would be tricky to code but has a chance of working. Any thoughts anyone?