Thanks for all the feedback. I think flagamuffin, rd95, and others are right that it could unnecessarily hurt the ability to share, and could be annoying for some current users on some devices.
Here's what I want to test: All posts from users without a hubwheel are invisible (everywhere) until someone with a hubwheel circledots one of that user's posts. Only users with a hubwheel can see these probationary posts (they will appear in global), and only if they can choose to see them. The author of the probationary post will be able to see it, of course.
There will probably be some small tag or something indicating that the user is probationary.