In the question index, questions that are unanswered are given a "missing" class so you could look for those to toggle the display of the submit button (and possibly a couple of text-display questions to explain what is happening).
Add something like this to the end of template.js:
$(document).ready(function() {
if($('#movesubmitbtn').length > 0 && $('#index .missing').length > 0) {
$('#movesubmitbtn').hide();
$('.boilerplate').hide();
$('.boilerplate:first').show();
}
else if ($('#movesubmitbtn').length > 0) {
$('.boilerplate').hide();
$('.boilerplate:last').show();
}
});
I would also remove the submit button from the index for the whole survey.
Add this to the end of template.css:
#index .submit {
display: none;
}