I started with the Neo Thunder source code, and after a hiccup with displaying sprites, I've got some rather hastily hacked-together source code that does the job. The stumbling blocks were defining palettes and tilemaps in the source code, as opposed to having the graphics tools create them for you automatically. But the upshot is, once everything is setup properly, the entire screen can be rendered as a single sprite.
The demo has two (2) modes of display; one shows 1:1 pixels and can be scrolled up/down to see the entire screen, the other scales the screen to the Neo Geo display. It looks a little less than awesome, but it's still playable. It should be possible to tweak the sprite graphics to have it looking as good as Sockmaster's Donkey Kong on the Coco, as in theory the scaling algorithm is deterministic and well documented.
|Scaled display on the Neo Geo under MAME|
Next step is to actually start on the port which will, of course, be 100% assembler.