This might be a stupid answer, but have you simply tried putting the value inside the declaration of the dijit.Editor?
For example:
<body class="claro">
<div data-dojo-type="dijit.Editor" id="myEditor">
This is the initial value of the editor
</div>
</body>
This will create the editor and set the value of the editor to be whatever text is inside the dijit.Editor markup ("This is the initial value of the editor").
If you need to programmatically change the value of the editor AFTER it has loaded, it seems like you can just use editor.set('value', 'some new text');
For example:
<body class="claro">
<div data-dojo-type="dijit.Editor" id="myEditor">
This is the initial value of the editor
</div>
<div data-dojo-type="dijit.form.Button">
Click to change editor value
<script type="dojo/method" event="onClick">
var editor = dijit.byId('myEditor');
editor.set('value','this is the new value');
</script>
</div>
</body>
I put together the working sample above in a jsfiddle
http://jsfiddle.net/Vu4NF/1/
this.replaceEditorContent(...)
from here without much sucess<div rows="10" dojoType="dijit.Editor" name="letter_header" id="editor_letter_header">