Sega never does something once. Which is why Cool Riders is such an interesting puzzle.
I'm pretty sure they only did that sprite compression once ;-)
but yeah, there's something weird going on with that.. I wouldn't be surprised if that port where it writes ou the details for the front tilemaps / sprites wasn't some kind of comms port to be used with the other cpu (which doesn't do much yet, unless it's really just for the linkup)
I couldn't understand it, nor does it seem to send enough data to tell me which parts of the sprites I need to decompress anyway. Really, I'm surprised Luca or somebody similar hasn't picked it up, that kind of pseudo-blitter weirdness is usually right down his street. (which is why I mentioned those 4 games to him)