Does anyone know if OpenWrt uses the hardware manufacturer part (xx:xx:xx) of the WiFi MAC address (xx:xx:xx:yy:yy:yy) to look up or select the appropriate radio calibration data for each target?
I change the first three parts of the MAC address in /etc/config/wireless, for example:
option macaddr '00:90:A1:CA:FE:05'
and am wondering if that is totally harmless with respect to affecting WiFi operation, or could lead to problems looking up things like the correct calibration data for a device?
No. The calibration data is stored by the factory in a partition of the flash which is treated as read-only by OpenWrt. In some models it is read during every boot up and in others it is only read on the first boot and converted to a file in the regular filesystem which is used by the driver.
Overriding the MAC address doesn't change the calibration.