Don’t know if you found help for this, but a basic option (assuming you know your PHP somewhat) is to look at another plugin which affects posts, such as Basic Bilingual (search for that) and look at how that plugin attaches an action to the edit post pages. That’ll show up at the bottom of the plugin. Then create a function which basically is a big echo statement of your message, and voilà! You have a message showing up at the bottom of the page.
Okay this could be more specific but hopefully this points you in the right direction…