Nevermind. I think I figured it out!
I make my SVG sprite a single column. Luckily all my icons are the same width, so this is one icon’s width wide: 40px.
I set css background-size to 100%, which works for IE.
The sprite fits the width of the containing element (extra height is hidden), and adjusts based on that element’s width.
The height of the sprite can continue to grow as the project rolls forward, as long as I never change the width and only add new icons to the bottom.
I control the background-position using only the vertical property. Horizontal is always zero.
Voila!
Want a smaller (20px wide) icon? Change width of containing element to 20px wide and cut the vertical position shift in half to change icons.

Just use background-position to center the background image. Like the following to center it horizontally in the top (if that’s what you need):
background-size: cover;
background-position: center 0;