How does the Spot Meter work? Are the central 4 pixels covered in different filters to allow for 2-color pyrometry, or have they otherwise been calibrated?

There are no filters used for the Spot Meter feature in Tau2. The center four pixels (only) are calibrated for temperature measurement. The in-camera processing can compensate for target emissivity, distance, and other environmental parameters, making filters unnecessary. This greatly improves image quality and sensitivity. These parameters must be entered over the serial command bus if they differ from the default values.