My senior instructor threw me a curve the other day, we have had alot of training in the Masanaga method and he brought up the fact that these days people are so caught up in the wonderful thought processes of Masanaga that they are forgetting that some masters of Shiatsu only treat the hara or don't even use meridians (oh my!) and that Masanagas method no matter how attractive is just a recent innovationand is still proving itself where the earlier building blocks of Shiatsu shouldn't be set aside because they have a much longer history of success. So I guess you need to keep the old with the new, it gave me food for thought.
The Qi can be influenced from anywhere on the body........... also in the energy body. The meridians, tsubos and Hara are all convenient access points and, as you mentioned, an entire treatment can be done via the Hara. Occasionally I find that after taking a Hara diagnosis, the touch (during diagnosis) has already influenced the Qi flow so that by the time I start on any given meridian a lot of the work has already been done.
It is also often the case that when, for example, you are working on a meridian on one arm by the time you start on the second arm there is very little to do.
I have been practicing Shiatsu, full time, for over 15 years now but I am still as amazed and excited by it as I was at the start of this great big learning curve.