Nevermind. I figured it out. I added the following to my custom.css:
@media (min-width: 768px) {
.t3-content {
min-height: 682px;
}
That causes the center content to stretch to the height of the sidebar I have there, but only when they're stacked side-by-side. At narrower screen sizes, they stack vertically and it no longer matters.
Please consider this ticket closed.