How to promote and encourage differently-abled participants?

One approach is to find potentially interested online communities via Google, Reddit subreddit search and so on, but then instead of rightaway mentioning your channel -- which can be interpreted as spam -- to instead either message the moderators, or first start a question thread, in which you ask if it's ok to mention your community. If they say no, you don't post, but if they say yes, you can go ahead (and might have gained more info on what posting style is appropriate, and the mods may now understand your good intentions).

Good luck!