As a matter of interest, a few moments of Googling finds this:
http://www.mail-archive.com/unicode@unicode.org/msg04506.html
Unfortunately, none of these scripts appear to have been Unicoded yet.
I suppose we could always use the Private Use Area in Unicode until someone gives an official assignment for the characters needed for these writing systems.
In particular note this from http://www.sciences.univ-nantes.fr/info/recherche/taln2003/articles/rocha.pd...
"The ISO Registration Authority has approved in February 2000, as an addition Standard, the alpha-3 code sgn to designate deaf sign languages, with country names added to identify their nationality. Thus, for instance, sgn-FR is the code for the French Sign Language, and sgn-BR is the code for LIBRAS, the Brazilian Sign Language."
In my opinion, XML encodings seem to me to be very undesirable as a way for human beings to encode anything. On the other hand, some shorthand for spatial encoding, similar to the a^b and a_b notations in TeX might be just what's needed.
-- Neil