Most of the battle has been learning the nuances of the Neo Geo sprites. I'm still unsure of how the sprite size field of the SCB3 register is supposed to work. I won't bother going into the details, but I still haven't ruled out the possibility that it's not implemented 100% correctly in MAME. I have more experimentation to do...
...but the upside of all this is that it has definitely been a worthwhile exercise with respect to learning Neo Geo hardware in preparation for the NGPACE project!
FWIW you can select COLOUR or MONOCHROME (GREEN/WHITE) rendering from the soft dip settings in the BIOS. There's probably a few more settings I could add at a later date.
I'll press on with the Neo Geo implementation, fixing the AI and adding the bells and whistles such as title screen, GAME OVER animation and high score entry. And as I've done with Donkey Kong, I'll burn a CD and verify that it runs on a real NGCD console!
EDIT: I've got the NGCD target building, and it runs far enough to show the title screen (place holder) - interestingly in the wrong colour - and then hangs. No further clues at this point.
Then I might look at reverse-engineering one of the most common sound drivers so I can add sound to the game, which I need to do for Donkey Kong as well.