You could use a "shortcode" type thing. Basically what you want to do is a find and replace on the Content. Use a tag/shortcode in the WYSIWYG editor like [UserForm id=5] and search for that string using RegEx. You'll need to load the UserForm and get it's render HTML into the Content variable of the page (not sure how to get the html of the user form off the top of my head though). Overload the Content function on whatever page you have.