I got a little confused about what you were saying cen, but are you trying to get at , for example deaf people communicate in a different way, they sign and this is recognised as its own official language (at least it is here in new zealand, not sure about the UK) and you are wondering why dyslexics who communicate in their own specific type of written language don't have this recognised officially alongside other forms of the english language such as UK,US,AUS and NZ?
well, I don't think i can say much as to why because i don't fully understand how dyslexia works, from what i think it is is people get letters mixed up in a word when reading or write down the sound of the word , rather than the spelling. but does'nt dyslexia change from person to person and there are different forms?
so i think it would depend if one dyslexic person wrote something, can another dyslexic person read that better than if they were reading something written out the usual way? because if that is the case , than i think yes dyslexic writing might just be a language of its own. do you find this to be the case with you cen?