I'm not proud to say that i can identify certain seiyuus by their voice alone. But hyosei and hitomi really have a distinct voice. And also a lot of vn turned anime still retain their vn seiyuu. I never would have thought gotou yuuko and itou shizuka are eroge seiyuus. Much less tomokazu sugita, katsuyuki konishi, nakamura yuuichi and fukuyama jun. They are all big name mainstream voice actors. Added bonus is Wakamoto norio. He voiced chousen in koihime musou too.
Btw, 3/5 of the SOS-dan are present. It's lampshaded in one of the end game skits. Maro-sensei is voiced by, surprisingly, ono daisuke.
And from f/sn, besides illya (monshiro) there is archer (gen), assassin (yoichi), rider (momoyo), kotomine (usami-sensei) and gilgamesh (kojuro).
Fans of steins gate would recognize lee as kurisu.
Toaru fans will recognize yoshitusne as misaka mikoto aka railgun.
Kyougoku from 3s is heero yuy.