Thanks! Sometimes I wish 24 colours was a binary thing though. 32 can feel like too much for some projects.
Anyways, I wrote a quick tool to peek around in the ROM. It seems like 8x16 px tiles, but, one bitplane is stored per ROM chip, i.e. not sequentially like on the NES. They're also sort of arranged in larger blocks (2x1tiles to make a 16px block). A raw dump seems possible. The twincobr.cpp mame github file has some addresses for palettes and clues that might be usable.
I was wondering why the tiles were rotated, as then only 1 bit per byte would be usable per scanline, but then I realized that the monitor sits on its side as it's a vertical game...
Edit: I managed to extract the FG&BG tiles, arranging the 16*16px blocks into 512*512px blocks. Each of those are 128KB as 4bp, but the foreground one will also need a 1-bit mask I suspect (unless dual playfield but that's beyond OCS afaik). Additionally there are some sprite and character ICs. Might be hard to extract the palette. It's likely set in a 555 format, then an attribute per tile or in the level data. Might be easier to just write an automatic colour associator using a few ripped tiles.
Edit2: Probably no more time for this. Plopped the WIP tile sheets in
The Zone. Edit: Seems Mame can bring up some palettes. Maybe F4.
Edit3: Couldn't keep my noodles off that cliff that was kinda jarring. It's a big asset so I rushed it. Didn't have the grays so I changed the detailing.
http://androidarts.com/Amiga/SharkHill.png