If I'm not mistaken, with the tool options window open, you can switch as you are creating it. I'm not sure about that though. There is also a curve editing tool whose name escapes me that can be accessed through the Edit Curves menu that can adjust the curvature of different parts of the curve.
Also UV placement will get f*cked up. You get a distortion near the corner. If you can overcome this obstacle on texturing you could have it done.
As far as I know, when dealing with 3rd degree curves, you won´t have a chance at good corners unless you click three times (point-snap) on the same point. It is a problemn inherent to Nurbs curves.
Another approach might be to stitch together two patches (One made of 3rd degree curves .. with the corner´s one made in 1 degree ones.) Be sure to follow the rules for Nurbs patching somewhere around the Birail posts in the Nurbs forum.
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