In the olden days Japanese scholarly works were written in 漢文, which is basically Classical Chinese. Together with a set of annotation rules (e.g. "read the next two characters backwards", "insert a particle here", etc.) it was possible to translate/transcribe the resulting Chinese text into Japanese.
Nowadays, it would still be possible to render Japanese in 漢文, but would require an easy translation. As modern Chinese is quite different from Classical Chinese, finding a translation from 漢文 into Chinese is usually non-trivial.
If the text you are considering was indeed written in 漢文, then you just need to consider how close a modern Chinese translation would be to Classical Chinese, which you probably know better than me. (As Axioplase points out, if the text was written in 万葉仮名, then "kanji-heavy" means very little and the answer is less obvious.)
I have read 五輪書 in English and a good part of it in Japanese. Any bigger book store here in Tokyo will have a copy in contemporary Japanese and most also carry the traditional text. I find the contemporary text easier to read, but the traditional text is also quite readable. Although 五輪書 is not written in 漢文 itself, the text is very close to the Japanese that would have stemmed from a 漢文 "translation" into Japanese. That is to say, a translation into 漢文/Classical Chinese would be quite faithful to the original text. How close that is to modern Chinese, I cannot say.
Personally, I didn't really like the English translation, so if I were able to speak Chinese, I would give the Chinese translation a shot before trying to read the English translation again...