This code will replace the "Add to Cart" button with a "Tell me when available" button when a product is out of stock.

You can setup the "Tell me when available" button to open a lightbox with a form or open a page with a form to collect emails addresses. In any case, create a page with a form and set the page URL to sold-out-waitlist.

If you choose to use a lightbox, we have a great Lightbox Plugin for this.