As I've been working on this for about a day, and finally got it working, I thought I'd share... so for those of you using QC 9.2 and onwards, here is a function for adding an attachment to a Test Step in the Test Lab - during runtime

function addAttachment() {

attachment = "C:\\Image.jpg";
// Interact with Step Factory of Current Run
// NB - this.CurrentRun is populated in my QC object constructor
var stepCount = this.CurrentRun.StepFactory.NewList("").Count;
var currentStep = this.CurrentRun.StepFactory.NewList("").Item(stepC ount);
var attachFact = currentStep.Attachments;

var theAttachment = attachFact.AddItem(null);
theAttachment.FileName = attachment;
theAttachment.Type = 1;