\definefontfeature
[arabic]
[mode=node,language=dflt,script=arab,
init=yes,medi=yes,fina=yes,isol=yes,
liga=yes,dlig=yes,rlig=yes,tlig=yes,
calt=yes,trep=yes,mark=yes,mkmk=yes,
kern=yes,curs=yes]
\definefallbackfamily[mainface][serif][Scheherazade][range=0600-077F][features=arabic]
and get output in which the letters are not joined up into words but printed separately, as in