This RhinoScript works on Rhino for Windows only.
Save the CurveSymmetry.rvb file to your hard drive and then drag and drop it onto a Rhino window. This will add the alias "CurveSymmetry" which will act much like a command that autocompletes when typed.
Workflow - Click on the end of the curve that you would like to match the other end to. Then select the symmetry plane you'd like to use.