The noun of "Jun Daijo tenno" doesn't exist at all, and moreover, it is correct to read it as a sentence, such as "Daijo tenno ni junzu."or "Daijo tenno ni nazurau." (Both of them mean that "to equate with Daijo tenno").
「じゅんだいじょうてんのう」という名詞はもともと存在せず、本来は「太上天皇に准ず」「太上天皇になずらふ」と文として読み下すのが正しい。