Code allows you to run flexible cord in a raceway for a short distance, in 'industrial' locations with 'qualified' maintenance. Not much help, I supppose.
The key, to me, is that the box still needs to be accessible. I encountered a situation like the one you describe in a house, where the garage had been converted to a bedroom. There was no way to access the box without tearing apart the hard ceiling- the attic of the house did not continue over the garage.
Yet, I suspect you're referring to 'drop,' or 'suspended' ceilings. There I believe the code needs some editing. We say 'no flex cord' above the ceilings, but we also now say you can't hang anything from the ceiling, either. (The last despite entire Caddy / B-Line product lines designed to help you do exactly that!). So, just where and how are we to transition? Looks to me that we've painted ourselves into a corner.