You can access the options with
$('input[type="text"]').autocomplete({
source: availableTags,
open: function( event, ui ) {
var autocomplete = $( this ).data( "autocomplete" ),
menu = autocomplete.menu;
console.log(menu.element.children());
}
});
menu.element.children() gives you the HTML List items. You can easily parse them (Strip the HTML) and build an array from that and pass it to your highlight function.
JSFiddle
Edit2: The following Edit does not return the expected values.
Edit: Instead of accessing menu.element, go for console.log(autocomplete.options.source);
New Fiddle