Quote - what i want/need to do is set a joint falloff 'heat' setting to be the same across a material.

You mean "material" as in "material zone"? I don't think there is any way to apply a falloff to a material zone. However with weight mapped joints, you should in principle be able to assign a value (say 50%) to a group of vertices that correspond to a material zone. I say "in principle", I'm not sure how you could do that in practice.