I am hoping most of your layout is using <div> and <table> tags. As Steven suggested, have unique id's associated with each of your block elements or any other elements that are an integral part of your webpage (maybe images).
Have an onclick event assoicated with the block/layout element to open up another window or a javascript pop-up, capture the element id and have a form on the pop-up where the user can submit their feedback.
I'd also suggest an onmouseover even associated with all these elements as well that display a tooltip indicating that the user can leave feedback about that layout widget/element.