I just did this through JAL and had success. As others have said, it was a pain. I had to first call AS to have them add the infant to the reservation and then call JAL back to make the payment. AS originally seemed to have put the infant in wrong so I had to make another call back to them. The JAL agent told me to specifically tell them to put the infant in as a "Special Service Request" and "INFT." Once I called AS back and did that, it seemed to do the trick for JAL. However, the saga was not over as they sent the info to their pricing team and then told me that I would receive a call back in 1-2 days. I received a call back in about 1 day and, of course, I missed it. So, I had to wait on hold another 45 minutes in order to get someone else on the line to complete the payment. The fee for business class was $370 which seems to match 10% of the best available business class fare + taxes. Hopefully some of this info helps others. I also seemed to be more successful getting through quickly to JAL right before 4pm PT. Maybe a coincidence, maybe not, but it worked twice in a few minutes vs. calling other times which took 45+ mins.