We definitely should be able to… Playtesting worked for me when I tested it late last year.
New Steam Playtest feature now live - Gaming - Most Epic Win
Don’t see why they would have removed SA as a “supported region”.
You sign up with patience. 
From 16:00 SAST on 8 September, visit the New World Steam Store page, and you should see a button similar to this:
Hit that and you’ll get an email notification when you’ve been given access, and the Store page button will change to something like this:
