Do you use "any old rock"? I know folks used to heat bricks like that as foot warmers (wrapped them in a towel).
I'm thinking that there would be some rocks that are better than others. Dense would retain the heat longer but weigh more and take longer to warm. Flat of course would be a big plus. Too porous and I'm sure there could be a cracking issue. Also how hot to allow the rock to get might be a deal if the chickens try to walk on the rock.
I'm thinking the rock heating would be more effort than benefit for me.