In Cubase, you can change the routing for L and R channels separately. I don't have Cubase installed at the moment, but if I remember correctly, there is a "channel settings" window that you can open, which shows more details about the channel than the mixer does. Somewhere there, you can select the output target of L and R channels separately. I can't remember though, it might be too awkward to do that.
Also, you asked about how to do powerpan in Cubase. I think if you right click you can choose between different panning modes (you need the full version of Cubase, though). The "Stereo Combined Panner" is similar to VSL's power-panning plugin. If you make both sliders very close to each other, you get effectively a "almost mono" signal, but you can still move both sliders simultaneously to "place the mono signal in stereo". This allows you to control both "stereo spread" and "stereo positioning" at the same time... for solo instruments its better to reduce the stereo spread a lot.
I guess mpower88 and I have different experiences about adjusting the stereo spread... I suppose both approaches are valid, depending on the mix. =)
And I completely agree that MIR is much better solution overall. But I'm surviving with Vienna Suite for now. =)