Insert a Link Using CSS

Not in a manner that will work across browsers. You could, however, do that with some relatively trivial Javascript..

function makeCasesClickable(){
    var cells = document.getElementsByTagName('td')
    for (var i = 0, cell; cell = cells[i]; i++){
        if (cell.className != 'case') continue
        var caseId = cell.innerHTML
        cell.innerHTML = ''
        var link = document.createElement('a')
        link.href="http://bugs.example.com/fogbugz/default.php?" + caseId
        link.appendChild(document.createTextNode(caseId))
        cell.appendChild(link)
    }
}

You can apply it with something like onload = makeCasesClickable, or simply include it right at the end of the page.