I looked a little bit into those two apparently non-Sega composers, Yoshimi Hishida and Shouko Ogawa.
Yoshimi Hishida => 菱田 吉美?
Credited here:
http://vgmdb.net/album/23294
Profile:
http://www9.ocn.ne.jp/~music-fr/credit/credit.html
Facebook:
https://www.facebook.com/esiecrew
Facebook bio says he has composed for video games, but he doesn't claim anywhere that he worked on Dark Wizard. Seems pretty likely it's him though.
Shouko Ogawa => 小川 尚子? (
Naoko Ogawa???)
菱田 吉美 and 小川 尚子 both wrote music for the film The Triple Cross:
http://ja.wikipedia.org/wiki/いつかギラギラする日
尚子 is readable as both Shouko and Naoko; maybe her name is misspelled in Dark Wizard? Or maybe they're just two people with the same name.