Is it permissible to eat after Shacharit or after Torah reading on Shabbat morning before reciting the Musaf service, and, if so, under what circumstances?
The Shulchan Aruch (286:3) writes that before Musaf one may partake of fruits, and even baked goods up to the quantity of a Ke'beitza (2 oz.). The later Poskim debate the question of whether the Shulchan Aruch speaks here of eating after reciting Kiddush, or without reciting Kiddush. Most authorities, including the Mishna Berura, Chida, and Kaf Ha-chayim, maintain that one must recite Kiddush before eating in such a case. Thus, for example, if a congregation schedules a recess for after Torah reading, before beginning Musaf, and people want to eat something before Musaf, they must first recite Kiddush. It should be emphasized that in such a case they may eat only as much as 2 oz. of bread or other baked goods. By contrast, the work "Chesed Le'alafim" held that the obligation to recite Kiddush takes effect only after the recitation of Musaf. Therefore, if a person wishes to eat before Musaf, he may do so without reciting Kiddush. (Of course, this applies only after one recited Shacharit; before Shacharit, one may not eat because it is forbidden to eat in the morning before praying.)
Halacha follows the majority view, requiring one to recite Kiddush if he wishes to eat before Musaf. However, Chacham Ovadya Yosef writes that the dissenting view of the "Chesed Le'alafim" allows one to be lenient in extenuating circumstances. Namely, if a person is ill or feels weak, and he has no access to wine, he may eat something to replenish his strength before Musaf even without reciting Kiddush. The same would apply to someone who served as Chazan or read the Torah in an earlier Minyan and must lead the service or read the Torah again at a later Minyan. If he feels tired and has no access to wine, he may eat before Musaf even without reciting Kiddush.
Chacham Ovadya reaches this conclusion based on the fact that this situation is one of a "Sefek Sefeika," or "double doubt." In addition to the debate between the "Chesed Le'alafim" and other Poskim, there are views that permit eating altogether before reciting Kiddush on Shabbat morning. Therefore, we may combine the lenient positions on both issues to allow eating before Musaf without reciting Kiddush if one feels the need to do so.
It must again be emphasized, however, that whenever eating is permissible before Musaf, one may not eat more than the quantity of 2 oz. of baked goods.
Summary: One who wishes to eat after Shacharit or Torah reading on Shabbat morning, before reciting Kiddush, must first recite Kiddush and may not eat more than the quantity of 2 oz. of bread or other baked goods. If one feels ill or weak and does not have access to wine for Kiddush, he may eat before Musaf even without reciting Kiddush.