Assuming one is using tinyMCE 4, you could add a placeholder upon init, and then remove it on focus. Remember TinyMCE uses an iframe.
Needs to be polished for being more efficient, but here is a quick approach:
tinymce.init({
//here all the rest of the options
//xxxxx
//Add the placeholder
setup: function (editor) {
editor.on('init', function(){
if (tinymce.get('Text').getContent() == ''){
tinymce.get('Text').setContent("<p id='#imThePlaceholder'>Your nice text here!</p>");
}
},
//and remove it on focus
editor.on('focus',function(){
$('iframe').contents().find('#imThePlaceholder').remove();
}),
})