Edouard Machery pointed out to me a sense in which "We need a MotC" is too strong.
Suppose you have a "mark of the visual", i.e. an account of the difference between a visual process and a non-visual process. Then, if visual processes are cognitive processes and you have visual processes that extend, then you have a good case for extended cognition without a MotC. (Of course, one still has some work to do to show that visual processes extend ...)
So, a "technical" refinement of the idea seems to me to be in order.