Scaling - Mirror about the x = y line

I’m trying to figure out the transform to mirror something about the xy axis, I tried this but it did not give me the expected result:

trmirror = Geom::Transformation.scaling([0,0,0], -1, -1, 1)

I could rotate the body by 90 degrees and mirror it over the y-axis but it seems like one should be able to do this with a single transformation.

Do you mean the xy plane?

Your first -1 mirrored about the YZ plane, and the second -1 about the XZ plane. Two mirrors cancel out. Although a 180 degree rotation would have occurred around the Z axis.