If you have included the JQuery simply put an ID in to your Body tag
<body id="MyID" ></body>
$("#MyID").html("<div>The stuff I want to add to the body</div>");
or without an ID you can just put:
$("body").html("<div>The stuff I want to add to the body</div>");
IfTrue is correct that you can't access the body with the code above from outside the Iframe, however the page that is inside the frame could contain the code to change the body.
As for accessing it outside the iframe you could use the following:
var ifrBody = $("#MyFrame").contents().find("body");
$(ifrBody).html("<div>My Stuff goes here</div>");
but only if you are within the same domain... you will get access denied if you try to do this across different domains. It is a security feature to prevent cross site scripting.