When creating a filled/masking region inside of a detail component family a work plane can’t be specified before, or during the sketch mode.  My work around to this is to create a simple rectangle as the region, finish the sketch and then edit the work plane.  Once the work plane is set outside of the sketch, edit the sketch and draw it accordingly.  The only real reason this becomes useful is the filled region needs to have a rotation value, setting the region to be on a reference line will make it less likely to break.