I thought that Marble Madness (aka satan's toy) did this with it's intro message on every level - that is, alternating data on every scanline. However it seems most emulators can't emulate this very well (I must admit FCE Ultra and Loopy both did). Thus well-timed code may only work for these emulators (or the real thing). Also, if you can get MMC3 every other scanline, you can still pull off some neat tricks I imagine.