This is to follow up on my post. I went ahead and put in Dynamat and Dynapad around the wheel wells and in all 4 doors. This reduced the noise somewhat. However, the truly bothersome pounding noise still will occur on really chewed up pavement (asphalt or concrete, usually torn up from studded snow tires and in need of resurfacing). Unfortunately, this is the condition of the road for my daily commute. At low speeds this is not so much of a problem. On decent pavement, the leaf is as smooth as glass. I have discovered that if I open the windows just a bit, the deep rumbling sound is less and it is not so painful on my ears.
All in all, the modification with Dynamat /Dynapad resulted in only a modest benefit, did not solve the problem, and probably was not worth the cost. (The same modification with Dynamat / Dynapad did shut down the noise from the same rough road surface in my Volvo S60 and that car is much quieter than the leaf even with the combustion engine.)
I think the acoustics in the leaf make it function more like a sounding board - such as in a guitar or drum - for the lower frequencies. This may be something the Nissan engineers may want to address at some point if marketing wants to make the quietness of the leaf a really strong competitive advantage. The car is otherwise whisper quiet. Higher frequencies are not a problem, there is only a breathy whistle when the pavement is smooth, and the motor produces a quiet high pitch hum at the most. People such as myself with hearing problems (and experts project that baby boomers will have a lot of hearing problems as they age) will find the low frequency noise in the leaf a serious dis-satisfier.