We have configured accessibility to only get all notification changes from the WhatsApp app. Nothing else. Although I should say you can have access to all events of you interacting with the screen: window changes, window touches, etc.
For the Android versions below 4.3, this is the only way to have access to the notifications. There is a much nicer (for the concerned user too) way to access them in 4.3+.
There's no need for a rooted device. We just read the notifications.
It's pretty straight-forward, except when you have messages from several contacts. There, the notification says "3 messages from 2 friends", so we had to use reflection to get the full text.
It's been a fun ride! 200 commits in 30 pull requests in 4 days for a total of 8k lines.
There's a few details on how we organized ourselves to hack this app together, but if you have more specific questions or comments, please shoot me over here!