Answer
(Fatwa: 1487/1170/L=1430)
The scholars of Fiqh have regarded completing the Quran in tarawih as mustahab on 27th night. The later scholars have approved collective dua too after the same; therefore doing collective dua after completion of the Quran is allowable:
وتکلموا في الدعاء عند ختم القرآن بالجماعۃ واستحسنہ المتأخرون فلا یمنع عن ذلک (فتاوی قاضي خان علی ھامش الھندیۃ 1:164)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband