Quote - What if you could set the option to mirror and then, if the plane is translated on X or rotated on Y or Z, a second pass would be automatically run, to create the mirrored loop in the same script run as the original?

Not sure I follow you. Do you mean just any transform, then the script automatically performs the mirror at the correct transforms? That would be fine. Or do you mean that the user has to perform the three transforms manually, then the script runs a second time? This latter would not really achieve anything worthwhile. The tedious part is mirroring the three transforms, and that's the part that would be nice if it could be automated. The only sort of symmetry that's really needed is left/right, and that is always achieved by those same three transforms mentioned above, all three are needed, except in the case where some of them are zero.

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Cross posted. Will try your new script.