The new design, with a single CAT4101 as the current gate, works fine with a single low-side N-channel MOSFET, and with a single high-side P-channel MOSFET and low-side N-channel MOSFET. Just have to finish breadboarding a simple 3 row x 2 column matrix of LEDs to test that configuration, plus write a simple program to drive the matrix. If it seems to work, then I'll build a new simplified low-side driver board with the new design. Should know by tomorrow...