I asked Bryan at the Digital Picture what he had seen. Here is his response.
I just AFMA'd three third party lenses on my 5D III. The Sigma 50mm Macro, Sigma 105mm OS Macro and Tokina 16-28 were all able to be setup for AFMA on my 5D III. The 5D III was not able to determine the serial number of these lenses of course. Best regards, Bryan http://www.The-Digital-Picture.com
Thanks!! Will try again now that I know it's not going to be a futile exercise