Yeah I know about the paper.
I suppose the ZBrush version is a bit different though.
Pretty sure that ZBrush relies on the dynamesh, it’s a natural fit for voxels.
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I actually thought about doing something similar.
You can get a (cheap) idea by using triplanar → visibility to “back” → then using lasso sel mask to do things.
My main concern was about resolving the whole “depth” thing.
Typically what’s happening at 49:05 in the ZBrush video, I don’t think he planned 2 heads 
.
Maybe there’s a shortcut to simply ignore the picking and start the new mesh at the symmetry plane (or middle point between the two intersections, a bit like the “edit-pivot” single tap thing).