You couldn't use the ROM as video memory. You could swap sprites in and out of video memory via DMA to the ROM (which you can also do on the Sega CD via the ext

You couldn't use the ROM as video memory. You could swap sprites in and out of video memory via DMA to the ROM (which you can also do on the Sega CD via the extended RAM) but you still had to load them into video memory to actually do anything with them. The video chipset was the real limitation here. Also I seem to recall the DMA trick was something that came alone a little after the Mega Drives release. Though I could be wrong on that. But I vaguely recall it wasn't something people knew how to do on day one.