I'm currently in the middle of a retopology process using the TopoBuild SOP. In all retopology software that I have used in the past, a z-bias can be applied on the retopo mesh in order to make it appear optically, but not physically, raised (superimposed) over the reference mesh. Otherwise, retopo mesh and reference geometry interpenetrate each other, and in such circumstances, it's hard to judge the shape and shading of the newly created retopo geometry.

All tutorials and resources that I've seen, recommend using ghosted or x-ray flags on the reference geometry. But ideally one doesn't want to work with semi-transparent reference mesh, because it makes parts of the retopo mesh, that would otherwise be occluded by the reference geometry - clearly visible. And if they're visible, TopoBuild will snap to them, which is, as you can imagine, undesired. Enabling backface occlusion doesn't help much, as TopoBuild still does see those occluded backfaces.

Documentation is also silent about this.

Any advice?

PS. I'd rather like to avoid using Peak or PolyExtrude SOPs to imitate z-bias.

The only acceptable workaround to this issue I found so far, is to use ghosted reference geometry in combination with viewport camera's far clipping plane set to a low value. This prevents TopoBuild from snapping to parts of retopo mesh that should be occluded by it, and maintains the optical effect of retopo mesh superimposed over reference geometry. Of course, the disadvantage is that for a zoomed out model inspection, far clipping plane must be manually reset.