Make a thread entry required in a workspace

Summary

How can you prevent an agent from saving/closing an incident if a thread entry has not been made?

Content

We're building a very lightweight workspace, where upon the selection of a certain topic, the agent is required to add a note in the incident thread, then save and close it. I'm trying to find a way to ensure the agent has entered a comment in the thread prior to saving/closing an Incident. Ideally using workspace rules. There doesn't seem to be a way to do this using the Rich Text Incident Thread object. I'd be very interested to hear if others have achieved similar functionality in their workspaces, and if so, how?