I think anti-aliasing level is fixed to 4 in Viewport Class in Yogurt3D v2.
The anti-aliasing level can be set by Context3D.configureBackBuffer() mthod.
So I think you may set anti-aliasing level following code:
Yogurt3D.instance.defaultContext.configureBackBuffer(width, height, antiAlias, enableDepthAndStencil);
I haven't tried this code, so I don't know if it works or not.