Salaams
The two volumes of Arabic font making by Stéphane Boeuf published by khtt.org are a valuable resource that deals with various aspects of VOLT. Not kerning specifically but they may help you figure it out.
http://www.khtt.net/page/31752/en
Hasan Abu Afash of Hiba Studio in Gaza is highly recommended as a real professional expert of Volting Arabic, including kerning. I am sure he can answer you despite the sad loss he suffered during the recent war - house damaged and brother killed in another incident...
http://hibastudio.com/
Vladimir