FriBidi contains a simple Arabic shaper. This shaper is based around the fact that Unicode contains codepoints for presentational forms of many Arabic characters, for traditional reasons. This can be great, as it allows very easy and simple shaping by analyzing the text, without doing any font-specific lookups. However, more advanced shaping features are not possible.
Just now I added support for FriBidi's simple Arabic shaper to libass. The bug about Arabic support contains a test case, and this renders great now.
Work to use a "real" shaper, i.e. HarfBuzz, is already under way, of course. :)