Well, I just learned the Menu (internal WP) system DOES allow custom classes! I just had to open the Screen Options (top right corner on Appearance > Menus ) and place a checkmark at 'CSS classes' ... Shows you can learn every day ;)
Anyway I tried adding the class fancybox-iframe to one of the menu items but on the page source code, that class is assigned to the li tag, not the a tag. I mistakenly did that in the example code in my previous post as well. But FancyBox needs to be attached to an a tag (or else be called via some click() trickery) so that is why it won't work.
Not out of the box anyway.
But I can tell you how to make it work if you are using Custom Menus:
1. Enable the possibility to apply CSS classes and add the class fancybox-iframe to the menu item you want in an iframe.
2. Open your theme header.php and add just before '</head>' the code:
<script type="text/javascript">
jQuery(document).ready(function($){
$('li.fancybox-iframe a').fancybox( $.extend(fb_opts, { 'type' : 'iframe', 'width' : '70%', 'height' : '90%', 'padding' : 0, 'scrolling' : 'auto', 'autoScale' : false, 'titleShow' : false, 'titlePosition' : 'float', 'titleFromAlt' : false, 'transitionOut' : 'fade', 'easingIn' : 'swing' }) );
});
</script>
Adjust the parameters like width, height and such. Now the iframe overlay should work :)
I quite like this... I might bake this right into the next release, even as a hidden feature. I can not promise anything but it is likely you can remove that addition to your header.php after upgrading ;)
Thanks sethneal, for this brilliant idea!