Still working on the software, but the hardware seems to work - passes a minimal RAM self-test and the ethernet chip links up. Nothing spectacular, just posting some pictures in case anyone's interested:
formatting link
So far, the only mistakes in the layout were getting a 3.3v regulator pinout wrong (it's rotated 120 degrees in the photo, but seems to work OK that way), and a wire to pull HOLD and RDY high (the m3a board pulls them the wrong way).
If anyone knows of a 24/25MHz oscillator to replace the 30MHz one on the M3A board (the blue blob just below the M32C chip on the left board), let me know - it would make the ram/ethernet timings near perfect.