Hi Pwood,
Welcome to SEBLOD. What you are asking is possible. This is how I would do it:
You could create a content type called "Message". The admin would use this to post their information to the user.
In the "Message" content type you would have a Select Dynamic field that gave a list of all the registered users on the website. You would also have a WYSIWYG field so the admin could enter a message. The admin could user the Select Dynamic field to click on the user that they want their message to appear to.
Then you'd have to create a Search / List called "Messages" and create a menu item for it. In the Search form, you would put your select dynamic field and set the live value to Joomla User > Int > ID. This will create a search of all the message that have been assigned to the logged in user.
In the Item view you would have your WYSIWYG field to show your message to the logged in user.
Make sure you set your Search Form to Hidden (do this in the menu options), and set the permissions of that page to 'Registered'.
Voila :)
_jrmo