I was looking for it and found something that might be helpful to you regarding this feature: The video that initially introduced me to the concept!
As you can see, there's an in-between step that makes it work as a normal (exportable👀) shader, which you may already be familiar with. It's easy to set pixel colors through code, so you could use the same trick to define the regions?
And I believe you can definitely make it work with an indexed palette rather than coordinates like you already have, though that could be interesting as well.