I can’t make work MaskLasso with BackfaceMask. MaskPen works okay and backfaces doesn’t get masked when BackfaceMask is on but with lasso it doesn’t work.
This is actually the correct behavior. MaskPen is the only option that actually works like a brush, meaning it is the only one that can be modified by Brush/Color settings. All other options (including MaskPen’s Rectangle mode) will not make use of Automasking.
The workaround is to hide the backfaces first, as these tools will only mask visible polygons. Polygroups (Group Front) can make quick work of this.