Tested out the new low-side driver design with a 3 row / 2 column matrix design, and after the usual screw-ups and bad connections, found that it seems to work mostly as it should. One oddity was a very brief transient spike in the P-channel MOSFET that caused a brief LED flicker in the second column in a row. Adding a one millisecond delay at a key point in software got rid of it entirely. I suspect it's due to a small difference in power source wiring in the breadboard vs. that in the controller box, and that the delay won't even be needed in the final controller version. But even if it is, 1 millisecond x 64 LEDs = .064 seconds = trivial increase in total data acquisition time.
Next step is to build a new version of the low-side driver board, to test it in the actual controller. I'll take pictures, and be ready to revise the documentation, if it does work. I really hope it does - cuts the cost significantly, and makes the controller easier to work with.
Unfortunately (or fortunately, depending on how you look at it), I'll be going to the Hackaday Superconference this weekend in Pasadena, so in all likelihood won't be able to test this out until next week.