Using hardboard for 3D printer enclosure; what is the temperature rating of hardboard?
Hardboard is called Masonite here in the States because that is the trade name of the product. If you look up the Material Safety Data Sheet you will see Masonite it states the following (Section 5):


  Auto-ignition Temperature (°C): >200 degrees Celsius


In Section 7, it states:


  These boards are flammable but difficult to ignite.


Furthermore in Section 10, it states:


  Conditions to avoid: Avoid sources of radiant heat and flame; and avoid sparks and sources of ignition in all electrical equipment, including dust extraction equipment.
  Avoid excessive build up of dust from boards.


The hot end works of printers are neither radiant heat, nor flames, nor sparks. Yes, you'll have a buildup of heat within the working confines of an enclosure, but if you are only using it for a base there should be absolutely no issues. If you were to build an entire enclosure from hardboard, you could put a thermal probe inside with the printer to ensure it doesn't get too hot, but realistically, it will never get hot enough within the enclosure to light the hardboard on fire.