How are analog thermocouple meters calibrated?
The effect of the resistance was not so great in comparison to the accuracy.
They were calibrated for a specific source resistance such as 10\$\Omega\$, and you would typically find that number printed right on the pyrometer scale. That's not the resistance of the meter, which is much higher. For low temperature ovens and plastics, around 30-50mV full scale and maybe 50uA as Tony says, so in the 1K\$\Omega\$ range.
The actual original installation of those things in serious industrial applications with their vacuum tube amplifiers and such like is a bit before even my time (we ripped out and scrapped a lot of them to install solid-state replacements) but I assume they would specify the thermocouple construction and the extension wire (if any) gauge to approximate the required series resistance.
I might add that printing a custom nonlinear analog scale with the associated markings, even in small quantities, was feasible in those days.