add an onMouseDown handler to check for your DIV via CLASS. if the correct DIV is being clicked, you will need to address the BODY element of the editor.
This took me for ever to find! enjoy
ed.onMouseDown.add(function(ed, e){
var body = ed.getBody();
if(jQuery(e.target).hasClass('someclass')){
jQuery(body).attr({'contenteditable': false})
}else{
jQuery(body).attr({'contenteditable': true})
}
})