Funny you mentioned that...

For awhile there I'm pretty sure I found a loophole that would allow me to upgrade an existing international B fare itinerary to biz class using miles, and not be charged for the miles. Similar to your situation. I'm not going into precarious detail on what I did, but I stumbled upon it by accident, and I didn't have to do anything but time my check-in and upgrade accordingly. Basically it has to do with performing this "upgrade via miles" at the absolute last possible moment. At least it was for me. That worked about 4 times for me, and i haven't tried it in about a 6 months or so.