You would probably have to modify tagcloud.js to customise the drilldown action. See the relevant but yet-to-be-modified code snippet below.
http://dev.splunk.com/view/SP-CAAAEME explains what preventDefault() does. Can't find much on the parameters for handleDrillDown().
events: {
'click a': function(e) {
e.preventDefault();
// Perform automatic drilldown on click on a tag
Drilldown.handleDrilldown({
name: this.settings.get('labelField'),
value: $.trim($(e.target).text())
}, 'row', this.manager);
}
},