Brilliant. This is actually very doable today even with a docinfo.html file. The words are currently added using CSS, keyed off the language in the CSS class. If you can sort out the CSS, we can push this in when font icons are enabled perhaps.
```css
.listingblock:hover code[class*=" language-"]:before {
text-transform: uppercase;
font-size: 0.9em;
color: #999;
position: absolute;
top: 0.375em;
right: 0.375em;
}
.listingblock:hover code.asciidoc:before { content: "asciidoc"; }
.listingblock:hover code.clojure:before { content: "clojure"; }
.listingblock:hover code.css:before { content: "css"; }
.listingblock:hover code.go:before { content: "go"; }
.listingblock:hover code.groovy:before { content: "groovy"; }
...
```