And here is a much simpler solution: Just sideload the free
Amiri font (I used amiri-0.108.zip) which has Italic Arabic glyphs, renaming the files:
amiri-regular.ttf --> Amiri-Regular.ttf
amiri-slanted.ttf --> Amiri-Italic.ttf
amiri-bold.ttf --> Amiri-Bold.ttf
amiri-boldslanted.ttf --> Amiri-BoldItalic.ttf
That is all, no patching or font modification needed (for firmware 3.19.5761 at least.)
Edit: Note that this font displays the book titles in bold because it has incorrect PANOSE weights. That would need to be fixed if you want to use it for reading KePubs, but it seems to work okay in the ePub reader.