Steg (i.a.),

Here is a link (based on the section in Bright & Daniels on writing
systems for transcribing movement) to a site that uses and gives
the basics of Labanotation.  It might be helpfull in transcribing the
signing component of a neanderthal lang.

Labanotation is primarily famous as a means of notating Dance
movements, but it is designed to be able to notate any movement of
any part of the body.  The book gives a parallel example of a text in
spoken Nakota and Plains Sign Talk.

Good Luck, and please share your results as and when they occur,