I'll just describe what I did to get Arabic book titles showing in the device library on my Glo (firmware 3.19.5761), because it might not work with all fonts:
I used the free
DejaVu Sans font
1. Open DejaVuSans-Oblique.ttf in fontforge
2. Use the Merge Fonts function to merge DejaVuSans.ttf
3. Change PS Names > Fontname from DejaVuSans-Oblique to DejaVuSans-Italic
4. Change PS Names > Name For Humans from "DejaVu Sans Oblique" to "DejaVu Sans Italic"
5. File > Generate Fonts to generate DejaVuSans-Italic.ttf
6. Open DejaVuSans-BoldOblique.ttf in fontforge
7. Use the Merge Fonts function to merge DejaVuSans-Bold.ttf
8. Change PS Names > Fontname from DejaVuSans-BoldOblique to DejaVuSans-BoldItalic
9. Change PS Names > Name For Humans from "DejaVu Sans Bold Oblique" to "DejaVu Sans Bold Italic"
10. Change TTF Names > Styles (Subfamily) from Bold Oblique to BoldItalic
11. File > Generate Fonts to generate DejaVuSans-BoldItalic.ttf
(I'm not sure if all of those steps are strictly necessary, but that is what I did.)
Then just sideload the DejaVuSans.ttf DejaVuSans-Italic.ttf DejaVuSans-Bold.ttf DejaVuSans-BoldItalic.ttf as usual and reboot the device. (Note that immediately after booting, the Arabic titles will still show as squares until a book is opened causing the extra fonts to be loaded.)
Edit: If anyone knows of a free font that has Arabic glyphs already included in the Italic variants, then it would be much simpler just to sideload that font.