Easiest would be to get OK for the idea from developers/web designer and to add a text input element in all your pages, which would NOT be used by tested system, but a placeholder for your information messages.
When you want to display a message to the tester, just locate this element and sendkeys message text to display it. Because tested system does not care about it, message will have no effect.
I would be against manipulating the HTML because I prefer to test the system as close to what will be deployed in production as possible. Having element for messages which tested system ignores in least disruptive.
For my tests, I just print such messages to the console, and also collect them in test object to be printed in test summary after all test finished running.