Inkscape

In 0.92 and 1.0, there's also a path effect that does this:
https://wiki.inkscape.org/wiki/index.php?title=Release_notes/0.92#Bounding_Box

Combine with Offset path effect to get a margin.
With Corners path effect, you can round the corners.

For a circle / ellipse, with the Corners LPE, use 'radius in percent' and set it to 50.

Thank you, Maren --

You've given me yet another reason to upgrade. I'm hoping that will happen in a few months.

In the meantime, I've learned how to write complex extensions -- not easy, since documentation is sparse. I had to "reverse engineer" existing .91 extensions. Anyway, the knowledge and experience I've gained may now enable me to be a resource to the Inkscape community.