The reports about Nushu get more and more confusing. Having read the popular articles and checked the few references in linguistics books here's what I think Nushu is.

Nushu is a syllabary - a writing system not a language. A syllabary is a system in which symbols represent syllables of the language - so, very often a consonant or group of consonants and a vowel. There are purer syllabaries (where close to each unique permitted syllable of the language has a symbol) and less pure syllabaries (where you create some syllables by combining other syllable systems and reading them in slightly compromised ways.

Nushu is used to write a language called Chengguan spoken by the Yao minority. Now, this is interesting so listen carefully. Only one Chinese language has ever been really widespreadly written - people spoke their local language but in effect they read and wrote (if they could) Mandarin/Bai Hua/Whatever-we-should-call it. So, these women were writing a language that wasn't normally written and that's why although Nushu is actually a writing system people have mistakenly refered to it as a language.

Even more interestingly I haven't been able to find out if Chengguan is a Chinese ("sinitic") language or a Kadai language (in the same group as Thai) or something else!

According to http://www.glossika.com/en/dict/ Chengguan is a dialect of Minnan (Southern Min) - the variety of Chinese that originates the southern part of Fujian province.

I am not in sociolingustics...just an idea.. but I propose -and think it to be necessary - to read again all those field research documents very carefuly...and especially the interview transscripts to be found there (see Google/"Nushu")focussing on constructivistic theory of communication:
.that is..communication is not transmitting "content"..communication is the task to "get into", "disturb", "reorganize" the "closed system" of the comunication partner
so that she will be able to "contextualize" what was the meaning/intention of the "message" , being "as such" only a matter of "physics" (sounds ect..)
..so..reading anew the interview transscripts of those Nushu writing women very carefuly...especially when they said.. they focused on being unable to transmit the "content" in ordinary chinese....
..I think that's the point...sort of a "closed womens comunication code"...and what's complicating the subject ..in a specific geo-historical context.