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.
Shulchan Aruch 89:4: "One who is thirsty and one who is hungry - behold they are in the category of the ill. If one has the ability to focus one's mind, one should pray. If not, if one wishes, one should not pray until eating or drinking."
Rambam's Mishneh Torah, Prayer 5:2: "...if he can concentrate, he recites the prayer; if not, he should not pray until he has eaten and drunk."
Aruch Hashulchan 89:24: "...even if he doesn't have an ailment at all, rather [he eats] in order to strengthen himself...even real kinds of food and drink, it is allowed."
From the above it appears that if one is very hungry he must eat before praying according to the Rambam. Beit Yosef 89:7 takes it down a notch and says that eating is optional and subject to one's discretion - "הרשות בידן." Mishnah Berurah 89:26 agrees with this conclusion even in light of the fact that today we are not able to concentrate during prayer even when not hungry.