I was waiting for @TimDown to update his answer with working code. But as he hasn't done that then I will post some myself (which is based on his answer).
The following function will return an array of highlight elements that have been creating, assuming the selection is still valid:
function GetAllCreatedElements(selection) {
var nodes = selection.getRangeAt(0).getNodes(false, function (el) {
return el.parentNode && el.parentNode.className == "highlight";
});
var spans = [];
for (var i = 0; i < nodes.length; i++) {
spans.push(nodes[i].parentNode);
}
return spans;
}