Usually if you are polite and ask them to carefully read the data from their computer (which almost always is the same as the timatic data), then they will figure it out. I had a similar issue recently when I was trying to board a flight in NRT bound for PVG without a Chinese visa. At first the checkin agent did not want to issue my BP, but after insisting, she patiently read through all of the documentation and found that I did not need a visa for my transit through PVG.
I'm guessing most agents get to the words "visa required" and stop reading there.