This little cluster will let you mirror any planar/non planar curve.
You have three kind of mirroring for non planar curves, according to the guide plane you need.
To add this user object, go to Files > Special Folder > User Object Folder & paste the .ghuser file into the folder. You can find the component under CalNic in the Grasshopper Plugins Tabs.
For feedback and suggestions text to : firstname.lastname@example.org