Hey Justin,
You can dynamically change the form endpoint with a small script to your generic Marketo form endpoint, which will accepts https connections. It would look something like this, though I haven't tested it, so I would pass this by your web developer before going live with it.
<script>
$.('#myForm').attr('action', 'https://app-***.marketo.com/index.php/leadCapture/save');
</script>
Just replace #myForm with the id of the Form on your live page and *** with the string that you see when you login to your Marketo instance.