Q. The Halacha is that one may not start a meal once it is 30 minutes before alos.
I would like to ask the rav shlita: May one forego this Halacha if there is a mitzvah for him to eat a seuda, such as one who awoke from his sleep shortly before alos and realized that he did not yet eat matzah on the first night of Pesach, in which case he has a chiyuv d’oraysa to eat matzah (since some hold you can eat it until alos) or even one who needs to eat melava malka?

A. Mishna Berura (89: 27) rules that the prohibition to start eating more than an egg-size of bread less than a half an hour before the alos mentioned in Shulchan Aruch (O.H. 652) in regards to the mitzva of lulav, applies equally to the daily reading of shema. Mekadesh Yisroel permits less than a beitza on the onset (Bein Hametzarim 2: 1- See question 347 in this forum).
Shulchan Aruch (O.H. 89: 3) permits eating when done for medical reasons. Mishna Berura (ibid. 24) explains that even if one is only partially ill it is permitted since his eating is not for pleasure. The same could be argued when the eating is for the purpose of complying with a mitzva. Chashukei Chemed (Suka 27a) addresses a similar question if one did not eat a kezais in the suka the first night until less than a half an hour before alos and rules leniently.
Horav Shlomo Miller’s Shlit’a opinion is that the mitzva of eating matza overrides the above prohibition. In regards to melava malka, he may eat less than a beitza amount.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a