Mirrored transformations on asymmetric geometries

I often work with models that have been completed and finalized by others, and have no access to or control over previous versions. X-Mirror often has no effect on these models.

It would be useful to me to be able to apply a transformation to selected vertices, but do a mirrored version of the transformation to any selected vertices that have a negative x-position. It doesn't need to be intelligent -- it doesn't need to understand the mesh or its symmetries in any way. Just if a vertex's x is positive, do the normal transformations; and if it's negative, do a mirrored version of the transformation.

Not sure how difficult it would be to implement, but it would be a life saver for me.