N.B. Sign languages, like ASL, are real, complete languages. Manually signed English, finger spell, lip reading, and the other various systems forced upon the deaf community are fake. Now that we got that out of the way… [And if you watch ASL, everytime the signer hits a fingerspell word, you feel like someone injected a chemical formula into a sentence, like dimethydiethlytetraisopropolyne-- a word so gargantian compared to the rest that it looks like a design error]
At various points in time people have suggested creating new languages organically, the same way that languages are created in contact situations, with a collaborative con-pidgin/con-creole. But only recently have a noticed that homesign– the ad hoc signs systems that are created by hearing family to communicate with a non-hearing family member are essentially ad hoc conlangs, or con-pidgins. And they have simple grammar! They have a dominate word order, etc. And ASL, Nicaraguan Sign Language and most of the other sign languages are also recently new sign languages, created from nothing once deaf children first got access to each other (although from current research we can assume that even they had a sort of real but simplified grammar in their home sign before they got together)
toki pona Sign language.
If I were to create a toki pona sign language, it wouldn’t be manually signed toki pona. It would be lexically constrained, but it wouldn’t use a system of particles: li, pi, e etc. In sign language grammatical particles are usually indicated by facial expression (which differs from the facial expressions we make to show emotion), and how you move the hands (e.g. repetitively, quickly, etc). A sign language also has a limited number of hand shapes that hands are allowed– these are like the morphemes of the language. They can also happen to be finger spell letters, but really the shape is arbitrary. Also, anaphora tends to be done by pointing in space, which establishes a prounoun that can be referred to later by pointing to the same space. This is way better than toki pona’s impoverished ona and ni.
So a toki pona sign language would replace e, o, pi, with facial expressions, e.g. pouting lips, wrinkled nose, etc. It would have a remarkably few legal hand shapes. It would likely be all single hand signs to keep with the principle of simplicity.
Just like written toki pona, it would be horribly verbose, but fairly easy to learn, and pretty hard to understand.
So if I were to make a toki pona sign language, I would probably make some reforms that could cut down on the verbosity.