I would have to say that although this sound was really prevalent in all of Asia for ages, I've never heard the term "freestyle" used (either the English word or the translation). It was called disco** at first, and later probably just pop.

** The sinophone world missed 70s disco, so what Chinese ppl think of as disco is 80s disco. I think this was less the case in Japan, but Japan never went for funk in a huge way, so.