The message can be set in your language files the same way as other translatable strings. The key is "/episerver/forms/viewmode/readonlymode" so you can either add the relevant XML to an existing language file or create a new one like this:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<languages>
  <language name="English" id="en">
    <episerver>
      <forms>
        <viewmode>
          <readonlymode>READONLY MODE! No forms for you.</readonlymode>
        </viewmode>
      </forms>
    </episerver>
  </language>
</languages>

Remember to set "embeded resource" in VS File Property... if you add the file for the first time. =)