Hunter, I think good athletics strongly encourage training in all of the above things listed. I don't think working with this is outside of any professional athletes knowledge.
As far as "external" martial arts go, I'm not sure I've studied any, maybe Kendo? When I studied Kendo, I would say most all of those things were discussed. In Subwrestling, BJJ, Aikido, MMA, Muay thai, Wing Chun, and any other art I've studied, I can think of people talking about these things.
As far as I know of "internal" arts I've studied- Those things are all important as well.
I have never studied any physical activity that didn't address the things on the list.
Well, I'm not saying that the topics of:
using ones own weight
the role of breath
the opponents weight/mass
are unique to internal martial arts and not addressed in external martial arts approaches.
Far from it, but the approach to how these are used are different. Take relaxation, most arts talk about it saying tension is bad, but my experience even with hachidan level instructors in various arts in seminars for teachers of those arts, provide little detail on how to use relaxation to train the body in the manner exposed by IS/IMA instructors (I've addressed this elsewhere, where I was told that such things were explicitly wrong, or that such levels of power were no longer needed or relevant....). Sure, my BJJ coach talked about relaxing to make yourself heavy, but then he didn't say how you could use that to connect your limbs to your center. I agree with much of Kevin Leavitt's comments posted later in the thread on this subject.
I would be more than happy to explore each of these subjects in a different thread as to how the approaches differ, but I will say this much, and I don't think its unique in my martial arts career. The above topics were addressed by external martial arts teachers to some degree, but never to the explicit degree I've experienced with some IS focused teachers, nor was the waza explained in such a way that working on the above principles was indicated to be the purpose of the waza. As I became little more experienced in working on IS, I could see that in fact, most of the high level teachers I had met had little understanding of this material other than understanding how to use the hips(but not initiating from the middle) which led to some rather amusing results as I have detailed in other threads.
As for kendo, despite my 16 years in an art that claims to be applying the principles of the sword, it has become something else entirely. Iaido is probably a better initial art to explore IS principles.