Sounds like a plan, but I wouldn't worry too much about using mortar as the gaps between loose laid bricks will fill with ash. A bed of dry sand will allow you to level them adequately.
Actually sand alone is too insulative with all the air spaces between the grains. You need to make the layer between the existing floor and the new bricks denser and more conductive. A 50/50 mix of powdered clay and sand is better because the much fine clay particles fill the gaps between the sand grains. it is better left dry to allow the free movement of the expanding bricks and also makes any subsequent removal much easier.