> Something similar happens in a lot other hardware as well, and you can solve it
> by using a z-buffer for just the sprites (optimise it so it only kicks in when
> needed and it will usually not impact performance much at all).

But it's not just a case of sprites obstructing other sprites or causing them not to get drawn. They actually appear over sprites that have a higher scroll-plane priority. It seems like z-buffering would need to take into account the plane tiles as well.

I really should investigate the Genesis again to see what actually happens. I think Charles has some documentation on his site.