Hi there!
I would like to know if anyone has been able to use the MaskPen brush in ZbrushCore.
Selecting it seems to have no effect. Ctrl+drag still gives the default MaskRect brush or any other mask brush that was selected previously. 
small_orange_diamond How is this brush supposed to be used?
The MaskPen brush uses the Freehand stroke. You need to start drawing actually on the model - if you start off the model then you will get the MaskRect.