No need to worry. I should be able to rescue the Arabic support in FreeSerif.
I had a look at the glyphs, and they looked really nice to me, so I
put in some more effort.
There were no tables for initial, medial, and final forms for
characters, but there is a pretty good listing here:
http://en.wikipedia.org/wiki/Arabic_alphabet, and I have written a
FontForge script that can conveniently insert the necessary lookups
from such a table of unicode characters.
There is still an issue with certain replacements, namely the "yeh" vs
"ʼalif maqṣūra" question mentioned in the above link. I'm hoping
someone familiar with Arabic typography will be able to help me out
there. Christian gave me some contacts.