To your point, I think that you could generate the duplicate geometry pretty easily but the issue would be making sure no overlapping sectors were visible at the same time, as that would cause glitches in the Build engine.

You might be able to subdivide the build engine's sectors into visleaves (borrowing terminology from Source) and clip all sectors in the mirror-verse according to the aperture + view-frustrum. To work correctly you'd need to do this every frame, probably an unthinkably huge task for the hardware at the time.