Joel Anderson gave a nice talk in CAPPE a few days ago on "Scaffolded Autonomy and the Extended Will". The talk focuses on the role of environmental triggers in facilitating the control of action, and on the ensuing possibility of an extended view of the will. The paper isn't online, but see his "Procrastination and the Extended Will" (co-authored with Joseph Heath) for the general idea.
Joel wasn't certain whether he wanted a real extended will thesis, or a weaker embedding thesis on which the environment plays a crucial explanatory role in the processes that support and sustain an (internally constituted) will. I think the key issue in choosing between them is whether one can get an Otto/Inga case going for the extended will, and especially whether one can get a twin case going, as one needs for a strong extension thesis. Something like the following three cases are my best stab at it:
(1) Ida is working on her book, which is important to her. She regularly is distracted with thoughts of watching TV. Every time the thought occurs she quashes it, and realizes that she should return to work. So she returns to work.
(2) Ollie is working on his book, which is important to him. He regularly is distracted with thoughts on watching TV. Every time thought occurs, he looks toward the TV, sees a sign saying "Return to work!", and realizes that he should return to work. So he returns to work.
(3) Twin Ollie (Ollie's duplicate, at least initially) is working on his book. He regularly is distracted with thoughts on watching TV. Every time the thought occurs, he looks toward the TV. There is no sign, so he ends up watching TV.
First claim: Ida is strong-willed in writing her book, even before the first thought occurs to her. Second claim: Ollie is relevantly like Ida. His veto process is externally triggered where Ida's is internally triggered, but this doesn't make a relevant difference. If so, Ollie is strong-willed in writing his book, too, even before the first thought. Third claim: Twin Ollie is weak-willed in writing his book. If so, being strong-willed or weak-willed can depend constitutively on the environment, in virtue of its role in driving cognitive processes. Thus, the extended will.
Obviously the verdict on Ollie is contestable, just like the verdict on Otto. Perhaps it would work better to choose a will-related property other than "strong-willed", which sounds too much like a global character trait: we're really after something more local ("focused"? better candidates?). And of course, acts of conscious volition needn't themselves be extended on this picture. But the key point is much the same as in the Otto case. Insofar as there are properties of the will that go beyond consciousness and are dispositional (as seems plausible), then in principle it doesn't matter whether the determinants of those dispositions are internal or external, as long as they play a relevantly similar role.