The section Generic font families does not say how to generically request a nastaliq font for the Arabic script. Anecdotally, the distinction of having a specifically nastaliq font is important enough in the Urdu context that Microsoft and Google bundle a nastaliq font with their operating systems for Urdu.

It seems to me that it would be good to explicitly specify which generic keyword means nastaliq, either by specifying that the cursive family means nastaliq for the Arabic script or by minting a new keyword for as was done for fangsong. (I don't know which option is more appropriate, but I'm guessing that using the existing cursive keyword should be sufficient.)