As if a stand in could even work. If I'm not mistaken sagiri does streams, wouldn't she talk to people then. So there's two scenarios first kyouka-san has a different voice from eromanga-sensei or second the voice a little girl would come out of a grown ass woman (If they dub over her for example).
In my opinion that can not work. Also it should be illegal to pressure an author who works under a pseudonym into revealing their face, especially if that author is underage. Without consent from their guardian it wouldn't be possible were I live although I have no idea how many rights kids/companys in japan have