Gas field in specified area, alternate...

But wait, there's more!
Another approach I've found on the webs, is to make a Gas Match Field, which makes a new field based on some other value.. eg. Density or Temperature.
You can actually specify a Density that is not the Pyro-sim's density, by using a Source Volume (turns out they don't always have to go into the last input). Point that to your custom volume
Finally, set the control mask to your new field.

All three of these nodes should be plugged into a single merge, which then goes into the Velocity Update of the pyro solver. (or the advect...)

I think this approach might be more flexible, if you are using it to refer to an internal pyro-value like Density or Temp?

Another approach I've seen skips the external source volume and copies the Temperature from the pyro solver itself. They use Gas Match to make a field which is "equal" in size/type to Temperature,
Next they use a gas calculate node to copy the Temperature to the custom field,

Finally, They  use a gas field wrangle to fiddle with the newly created field. Normalise values, or do whatever..

They use the custom field as a mask for dissipate & turb.
Quite a neat way to mess around with the temperature.