Basically, I'm using a Code Block with custom HTML5 for the structure and CSS (LESS) for the styling.
However, you could still make this happen on an Image Block with the caption set to overlay on hover and just use CSS to override the default styles. So, if you add a link in the caption you could style to look like a button. Below could be a good starting point to override the styles and get things in place for styling with CSS.
text-align: center !important;
top: 0 !important;
// Project Type Styles //
// Project Title Styles //
// Project Link Styles //
Hope this helps and points you in the right direction.
Devon Stank @ Devonstank.com (Squarespace Tips & Tricks)