I get a sense here, Jonathan, that you are overcomplicating a situation which is really only useful to the practical photographer as a guideline. I'm sure there are far more precise scientific methods of determining pixel quality without eyeballing the legibility of small text.
Anyone who employs your method is giving herself unnecessary work, because it's the performance of the whole sensor that's important.
If we take Marc's second set of P30/5D shots where he's got the focussing better (on another thread), we can see that the P30 delivers more detail in the shadows in same size images, by virtue of the fact it has a great deal more pixels, yet there seems to be some doubt as to whether the P30 at the pixel level has any more DR than the 5D.
Your method of shooting the chart would be useful in this situation, to shed light on the performance of these two cameras at the pixel level. However, in the two equal size crops below of the dark cupboards and drawers, it's clear that the 5D image has unacceptable noise but the P30 image would be acceptable as a print.
If one were to substitue the image of the drawers for your chart, it is easy to imagine that the P30 image would reveal any text more legibly.