I cannot say for certain if there have or have not been, but frankly, I don't think anyone in the "royal family" but the king has had an original thought since Christianity came to Japan, and not even the king himself since WWII, either one. So I find it extremely unlikely that there was even a secret convert.
And that lack of original thought is a big reason why I have no respect for the so-called "royal family" of this country.