Add alternative "access denied" text

I'd love to have a text area to put in something other than the stock "access denied" message. Ideally this would be a text editor that appears in the "access" area of the edit screen for each node. Allowing use of TinyMCE on this text area would be fantastic.

I may be able to help with this, depending on workload over the next month or so. Pointers to the key spots in the current source would be greatly appreciated.