In order to fly, EM needs to embrace a key feature supported by functionalist theorizing namely multiple realizability. A little philosophical history will help here. Functionalism (in its non-extended form) freed physicalist philosophy of mind from a kind of neural chauvinism. If our mental states were constituted by their functional roles, and the material contribution of our brains was merely implementationa I in character, then robots, Martians, Klingons and gaseous creatures from the outer limits of the universe could all join us in having mental states, just so long as the physical stuff out of which they were made could implement the right functional profiles. Stretching the word' skin' to include boundaries made of tin and gas, traditional functionalism bequeathed to the mind what we might call within-the-skin multiple realizability. And within-the-skin multiple realizability requires within-the-skin implementational materiality. But now extended functionalism merely plays out the same logic beyond the skin. If the specific materiality of the substrate doesn't matter to cognition, outside of the fact that it must be able to support the required functional profile, then what, in principle, is there to stop things-beyond-the-skin counting as proper parts of a cognitive architecture? Nothing, that's what. And this beyond-the-skin species of multiple realizability, which is just another way of characterizing the core philosophical commitment of EM, requires beyondthe-skin implementational materiality. If we look at things this way, the really radical and revolutionary movement was functionalism, not EM. EM simply makes manifest one of the implications of functionalism. (Wheeler, 2010, 33).Now, I think one has to be careful here. One has to draw a distinction between a modal EC claim and a non-modal EC claim (as Wheeler now does in his Extended X manuscript). The modal claim, essentially, is that it is (logically? nomologically? metaphysically?) possible for cognition to extend. The non-modal claim, essentially, is that cognition (actually) extends. Functionalism makes the modal claim at least pretty plausible, even though one can raise objections about the nomological and metaphysical cases. But, functionalism, it seems to me, does not warrant the claim that cognition is (actually) extended. Functionalism allows for multiple realization; it does not lead to (guarantee?) cases of multiple realization. And, even if functionalism did entail actual multiple realization, it would not thereby entail actual multiple realization outside of brain. Maybe it would only entail MR in brains.
As an aside, Carl Gillett and I have argued in "Levels, Individual Variation and Massive Multiple Realization in Neurobiology", for example, that many cognitive processes are actually multiply realized (or more technically, multiply implemented) in the brain at many different levels, but I don't see that cognitive processes are realized very often, if at all, out side the brain.
All of this is pretty rough--not what I would put in a journal article, but I think the lay of the land is clear enough for a blog post.
Aizawa, K., and Gillett, C. (2009). Levels, Individual Variation and Massive Multiple Realization in Neurobiology. Bickle, J. (Ed.) Oxford Handbook of Philosophy and Neuroscience. (pp. 539-581).