Reading your post, it sounds as if while the approach is being flown and let's say that the OAT is ISA -20ºC, the pilot has to calculate the actual MDA. Is this correct?
Yes, or rather calculate the indicated MDA corresponding to the true MDA. And they should be applied to other minimum heights in the procedure. Correction tables are available in e.g. UK AIP AD 1.1.2 para 3.1.3.
It sounds as though you're familiar with that. And it's not a correction of the QNH as such, rather a correction for the limitations of the pressure altimeter.
The only sense in which QNH could be described as being "corrected for temperature" is if you consider it to be the sea level pressure "corrected for temperature". But that's putting the cart before the horse since QNH is measured and SLP for plotting on charts is subsequently deduced from it.