Interesting that Celani used this same method.
That's probably where the idea of switching from the LT Ecat to the HotCat came from.
it is in fact perfectly good when used with surfaces that approximate grey bodies, or when properly calibrated at temperature.
A method that gives you an estimation of the heat flux proportional to the 4th power of temperature is only good for mixing up the results. Just the switching from a LT device and a mass flow calorimetry, to a HT device and such an indirect method, is quite suspicious.
If Celani followed good practice IR measurement protocol, calibrating surface temperature against camera reading at all the temperatures used, I see no problem using this method.
Celani used a few thermocouples placed on the glass tube of his cell. His method was even more simple and direct than using an IR camera. Notwithstanding, MFMP got a lot of problems when they tried for a couple of years to replicate the Celani's results using his method. At the end they decided to build a mass flow calorimeter.