I recall seeing a way to do this that involved taking out the stem bolts and threading them through the wrong way with a penny or similar in the gap which the bolts pressed against and opened up the bar clamp area :? I think you'd still only get a low rise bar in though.
I used the penny method just as you described and managed to get a set of 2" azonic risers in no problem! But that was back when I weighed 10 stone and didn't care about stressing a product out too much!