Inferred Task dependencies using Properties of simple types

What you are trying to do does not work because Producer.message is not a task output. While Gradle has the concept of input files and properties, it does not have output properties, only output files.
In example 5 of the page you referenced, Producer.outputFile is annotated with @OutputFile and therefore the producing task information is carried along with the Property instance. The example maps the contents of that file to the consumer’s input property. Perhaps you can adapt your code to use an intermediate file in a similar fashion.