Why can't you mirror projected geometry in a sketch?
Try it: project a curve or a point from elsewhere in a model, into a Sketch, then try to mirror that projected geometry inside the sketch. Can't do it.
Yes, this is true, but I can offer a workaround. After you Project the curves/edges into the Sketch, convert them to Reference Curves and then Offset them zero distance. Now Mirror the zero Offseted curves.
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA