If you add "<!-- " to the end of the value of the $$HTMLHead field, and add "--></HEAD>" at the very top of the form, you'll effectively comment out the Domino-generated <BODY> tag by making it look like a comment in the <HEAD> section..
It ends up looking like this:
<!-- </HEAD><BODY> --></HEAD>
thus leaving you free to add your own <BODY> tag.