We've got a nice 8" fireplace hearth here that serves well for step-ups, so that could be a good option, thanks!

I ended up with balance problems on the rear-foot-elevated lunge also (which is how I ended up balancing my foot against the Wii balance board). But the way the Bulgarian split squat is shown in rydra_wong's link above may have fewer balance problems. I'll give it a go.