Maybe the ISAR (Foundation for Research on Islamic History and Art and Culture) might help you out.
In 1999 Osman Selaheddin Osmanoglu published "The Ottoman Family" for the 700th Anniversary of the Foundation of the Ottoman State. Unfortunately, the copy I have does not mention the mother's name of the V Sultan.
The address they give in that Publication of ISAR Foundation No:9 is:
P.O. Box 24 Besiktas
80692 Istanbul - Turkey
Will try and find out if they have an e-mail.
Hope this might help.
Nadia
Terry, I also found on the internet: "Ottoman Empire-House of Osman" and they get their data from the French "Almanach de Gotha", which is well known as having the correct data and their e-mail is: almanachdegotha.org/id127.html.
The Almanach says that Memed V father was Abdelmecid 1 and the mother Gul-Cemal Kademefendi, and on that information I did the correction on my tree.
Thanks for bringing up that query.
All the very best.
Nadia