I see what you mean now, but I don’t think I can add this to the tooltip functionality – mainly because the placeholder (like you pointed out) is only available for input and textarea elements.
You can use this function to add this placeholder:
function addPlaceholderText(fin,text){
jQuery("#dffs_"+fin).find("input, textarea").attr("placeholder",text);
}
// Call it like this
addPlaceholderText("YourFieldInternalName","Placeholder text here");
Alexande