Thanks Wayne, the apple script is an interesting idea. Unfortunately, no one on my team has experience with it. I'm often surprised at the limitations of numbers given it's many awesome features. It could be as simple as a "bring to front" layering comand if there was such a thing. Any other ideas, anyone?
Here's an example you might find useful. It uses an image fill in the cell, and conditional formatting to replace the image with white fill (and text) when a specified condition is met. The text meeting the condition is here controlled by the state of the checkbox in the cell to the left of the one containnig the image fill.
Cell B2 contains the formula shown above the first image.
Victorian piano image from The Graphics Fairy