You will not be able to directly edit the body tag when the page is template derived. I think the easiest way to make this edit would be to just add in a script reference somewhere in the head editable region, then just put in the js code you would normally have in the body onload event just inside of the script tags like this:
<body onload="some js code here;">
some js code here;
This should accomplish the same thing and give you the same result. I'll update your other post with a reference to this one, please only update this post.