How can I define an improper rotation between frames in SimMechanics?

I have a complex CAD mechanism created in Autodesk Inventor, which I have imported into simmechanics as a system of interlinked joints and bodies (and refined/stripped back significantly in the process). I would like to create another, identical system, mirrored in the y-z plane.

The simple way of doing this is to copy the original multibody system in Simulink, change the orientations of all the parts in the copy to be dependent on one key frame, and transform this frame to create a reflection in the desired plane. However SimMechanics does not accept 'improper' rotation matrices (ie where det( R) =-1) as frame orientations.

Mirroring the system in Autodesk and re-exporting/importing into Matlab is not an ideal solution, as it will take many hours to simplify and verify the imported mechanism - I was wondering if anyone could think of a shortcut to mirror the orientation of a rigid body in Simmechanics.