What I'm actually trying to achieve is a hack for modeling with trilateral symmetry. I'm trying to achieve this with the following modifiers:
Starting with a model like:
1 - Mirror Modifier (mirror in X and Y) to give 4 identical quadrants. This gives me something like:
2 - Boolean Modifer (use a big cube to remove the 2nd quadrant i.e. upper left). I only allow the cube to draw as wires so I can see through it. This give me something like:
3 - Some sort of deform modifier (this is the crux of my question) that will close the shape in some reasonable way to achieve trilateral symmetry and allow me to continue modeling only in the original "quadrant".
Simple deform doesn't seem to be the right thing here.
Mathematically there are a few ways to do this, though scaling everything along the circle seems to be most natural and gives me the result I want. So I could implement any of those transformations in Python as a one off. But I'd rather hear someone experienced with Blender say that one of the deform modifiers does the trick for me.