I need to raise the temperature 20 degrees for one type of filament, but there is no way to do it other than watching the temperature and quickly reacting whenever the gcode sets it.
We can't really say much without any information about your setup or the systeminfo bundle. Temperature offsets work just fine for me so there must be something in your setup that is causing it to not work. A test in safe mode would exclude any 3rd party plugin from being the issue.