1) Put editor in advanced mode by clicking on red arrow on top right (you have to be in advanced mode to override a variable)
2) Click 'Variables' link (now it is possible to 'override' the missing Head-tags variable).
3) Override the 'Head-tags' variable by clicking the "Define New Variable" button (even though we want to override)
4) Now you can paste in a tag like this: <link rel="shortcut icon" href="http://enter-url-here-to-your-store/favicon.ico type="image/x-icon" />
I love yahoo store, but it has some serious usability issues right now ;)