Many people like to talk about "energy disruptions" and the like within this field and it is appropriate to a degree. And, as I stated earlier, this discussion of scarring comes up often. Generally, people will say they interpret the scar as a disruption, yet, clinically and scientifically this is only valid if someone actually develops symptoms related to the meridians/areas that are scarred that can be linked to the scar itself and not the procedure done within. After seeing many patients and a very broad range of cases, I've yet to see someone with a condition that started as the result of a scar. A condition as the result of a surgery is another matter, but from a scar, no.
Acupuncture is much more than just the flow of energy through the meridians and an interpreted disruption in the flow is only part of what leads to health problems. In reality, we know that needling has many more effects than on the meridians alone - there are changes in brain chemistry, hormone production, releases of various proteins, etc. and most of these, based on clinical research, are responses from single points (i.e. not related to manipulating and entire meridian).
Will dense skin effect needling sensation, yes. But does needling sensation, or level of, actually effect clinical success in treatments? Well when you compare something like Japanese acupuncture and standard TCM you would have to conclude probably not. Might you choose different points to use, etc. because of a scar? Yes, of course, but the fact remains that you will not see a condition - be it physical, psychological, etc. - develop purely from a scar. Intrinsically, then, it is somewhat of a null argument.
All that said, you can (and should) help the scar heal with acupuncture and there are many methods. One I use frequently is the "turtle" method as described within my Japanese acupuncture section on using moxa to treat qi/blood stagnation as it appears in masses, scars, etc. - here.