can a system plugin display error message?

[helives]

I've written a simple access control system plugin that blocks access to certain areas of the site based on privileges i've stored in the db.  I block the access by changing the task in "onAfterInitialise()" from edit or save or new to nothing, but I want to also display a message on the page when it loads that says something like "you do not have access to edit this page"

How can I add this kind of error message to the page from this plugin?  I saw that I could set the message from a controller, but this isn't a controller... help?

thanks in advance