Yosuke Okunari confirmed Nakabayashi composed the music for Dunk Shot in the booklet of SEGA SYSTEM24 SOUND COLLECTION (see
http://vgmdb.net/forums/showpost.php...19&postcount=4).
As for Alien Syndrome, I'm not entirely sure how we know Nakabayashi did the music for that game, but I've seen lots of site pages that say he did its music. Maybe someone found out from a Sega staff member?