but I don't know why I can't use the face of the plate or the face of the air at the plate as a port.  Can you explain that?

I'm not sure why but I'm guessing it's to do with how it calculates power. Experimentally, two microphones are needed to distinguish between incident and reflected waves.  

As you've noticed, the transmission loss values is determined by the boundary condition and size of the plate (resonance of the plate). I believe that is why this method is typically used for specific mufflers and fibrous materials as opposed to general sound barriers where transmission loss changes with size and boundary conditions.

Kind regards,