Fallback to Farsi?

This is very unlikely. Farsi has just in common of being written with the Arabic script, but Urdu is in fact a language extremely near from Hindi (up to the point that for long it was considered the same language, with just a different script, and transliterators have been developed). In fact even today there exists way to transliterate Hindi to the Arabic script, and Hindi would be a better fallback.

Farsi is too far from Urdu, and most speakers of Urdu are living in India. Those living in Pakistan would not use Farsi anyway, they would use Pashto (another Indo-Iranian language near Farsi, written with the Arabic script).

So I think the correct fallback chain is still:

 ur > hi > ps > fa > ar

You could reverse it for Urdu speakers in Pakistan if they use a more specific "ur-pk" locale:

 ur-pk > ur > ps > fa > ar.

(I could integrate "hi-arab" in the chain but for now we don't have transliterators from Devanagari script to Arabic script, or the reverse; the behavior of vowels is very different in the two scripts and cause difficulties)

You could apply the same for the Kashmiri language (for which a development of transliterors between the Gurmukhi and Arabic scripts is highly desirable, nad may be too with the Latin script, and there's alsready transliterators between Gurmukhi and Devanagaru scripts : the question of the script for that language is still not decided locally)

Thanks for the information but the translators decide, so we'll only rely on their wish.