Bounds (pp. 89-91) has a number of quotes which at least suggest that there is a relatively short path from observations of causal environmental influences on cognitive processes to extended cognition processes, but how about reviewing a longer bit of text by Clark and Chalmers? In the early sections of their paper their appear to be two lines of reasoning meant to support EC. One is based on causal contribution; the other is based on some sort of "cognitive equivalence" between intracranial processes and transcranial processes.
So, the last sentence of section 1 is "We advocate a very different sort of externalism: an active externalism, based on the active role of the environment in driving cognitive processes" (Clark & Chalmers, 1998, p. 7). One plausible interpretation here is that there is a coupling-constitution argument being given here.