I want to have a hidden div on the page with the elements of a SimpleModal dialogue, but when I set the CSS visibility to hidden and fire the modal, it is empty.
How can I use a DIV in this fashion without making it visible on the page?
Instead of visibility: hidden; what you want is display: none;. Most jQuery plugins work off display instead of visibility (even more core jQuery functions related to showing/hiding do this).
SimpleModal (and every other jQuery modal I know off) will reverse display: none when they show the modal...they're actually expecting this :)