Are there any accepted authorities of Halacha that permit a man to eat (i.e., "real" eating such as consuming cake or cereal) before davening Shacharis under "normal circumstances" (i.e., a healthy person with no other reason to eat then other than "they are hungry")?
It says in Hayom Yom for Yud Shvat that when Rebbetzin Rivka (who was the wife of the Rebbe Maharash, the fourth Lubavitcher Rebbe) was eighteenth she felt ill. The doctors told her that she must eat breakfast. She didn't want to eat before Davening so she started Davening early and afterwards eating breakfast [I assume that means that by her normal wake-up time she was already finished davening and could eat].
Her father-in-law (The Tzemach Tzedek of Lubavitch) said "A Jew must be healthy and strong..." He then said "You should not be without food. It's better to eat for the sake of davening than to daven for the sake of eating".
In general, in chabad one learns chassidus before davening which results in shacharis starting 9:00-10:00. Therefore, the custom in Chabad is to eat before davening and in most (if not all) chabad yeshivas, mzonos is commonly served before davening.
This reminds me of a Rabbi that I knew who used to say "Better to eat and think about the Davening, than to Daven and think about eating".
The Rivvivos Efraim says there is no Heter.
However, HaRav Ben Tzion Abba Shaul in the notes here mentions based on Orach Chaim 89:4 that if you are hungry or thirsty and can not have Kavana properly then you may eat before Davening.
Rabbi Herschel Reichman said that in his time at a Belzer yeshiva, Chumash was studied before Shacharit. During this pre-shacharit learning, cake was served.