If it is practical to pre-create the post for each author, you could accomplish your goal as follows:

1. Give them a WordPress role of Subscriber
2. Create their post and assign them a Post Editor role for it

If you need to let them create the one (but only one) post, I would need to write a few lines to interrupt the post creation form and processing if they already more than the allowed max number of posts. At this point, I could implement this as a hidden Role Scoper option (activated via constant definition) for $50 to $100.