I think I mentioned this before somewhere, but one of the exercises Tohei Sensei described in "Ki in Daily Life" is actually a breath/fascia exercise, although he doesn't say that clearly. Someone else posted the comment that Tohei attributed that particular exercise to Ueshiba Sensei.... that would make sense, since it is essentially a power-gung.
Regardless, it doesn't hurt people to look around. Ueshiba certainly did.... Aikido is founded on the Ki of Earth and the Ki of Heaven, the famous Yin-Yang dichotomy.
I have the old version of that book somewhwere. WIthout looking it up again, I remember his methods not discussing what is really being trained at all. I just don't feel like finding it. FWIW that exercise is not a Ueshiba "invention" or discovery. He got it from Takeda. It's part of the DR in/yo ho breath-power method. It's mentioned briefly and incompletely in Certa's new book on his training in mainline DR, with a picture of Tokimune Takeda doing it-think seated in seiza. Which is not a good way to do it IMO. It's interesting to read and hear those of the mainline denouncing certain aiki/Ki training. And then they publish a picture of their head chef doing just that. It would be fascinating to see just how many of any given art actually knew anything at all about the internal workings of their own art.