Thanks a lot Niall, for your answer and the interesting quote of D T Suzuki. Yaron told me about this book and it seems I should find the time and read it.
I wonder if it's something to do with the translation. In Japanese that point is usually called mizu ochi or mizo ochi. In English we say solar plexus though, from latin - network of the sun.
A case of miss-translation is quite possible, although it is worth mentioning that in several occasions Tada sensei did correct the translator, at least the translation to German (The same person did the translation into German and English. It was truly amazing)
It seems to me that it's possible that this name to the solar plexus may be derived from Tada sensei's influence from Tempu Nakamura sensei. I should ask Yaron regarding this possibility.
BTW, the solar plexus is called in Hebrew "maf-te-a-ch ha-lev", which literally translates to English as the key of the heart.