Is it permissible to allocate a portion of zakat funds to projects such as building mosques and Quran memorization centers, or homes for marrying orphans, after the vast majority of the zakat has been distributed to its rightful recipients as prescribed by Islamic law?

Zakat (definition) has legitimate recipients stipulated by Allah Almighty in His saying: "As-Sadaqat (Zakat) are only for the for the poor and the needy and those employed to collect (the funds); and for bringing hearts together (for Islam); and for freeing captives (or slaves); and for those in debt; and for the cause of Allah; and for the traveler (a son of the road): A duty imposed by Allah. And Allah is All-Knowing, All-Wise." What is mentioned in the question is not among the recipients of Zakat, and paying a portion of Zakat to other than its rightful recipients does not fulfill the obligation. Rather, the entire Zakat must be paid to its legitimate recipients.