Sometimes it’s right in front of us. The solution, from a very nice person on the css-discuss list, was this:
Use a background image in the centered container the same width of the right column, placed right, repeated vertically.
In my case, that would be not the big container, but the one I use to hold both the main content area and the sidebar.
It works in both these conditions:
It really works. Crisis. Averted.