I mean 256x224x8bpp@30fps is just plainly impossible no matter how much hardware you throw at it. The only way you'll get that is with a 32X style link where the SNES video out goes into the add-on unit, and the add-on unit outputs its own video signal that super imposes one device onto the other.But full-screen (256x224, 8-bit, @ 30fps) doesn't seem like a possibility without additional hardware.
The only thing that prevented something like the MSU1 existing in 1998 was that the cartridges would be the size and cost of Neo-Geo games. I don't think "marginally better audio quality and four minutes of cutscenes" would justify someone to pay $400 instead of $60 (in 1998 money) for a game ^^;You got that wrong. MSU-1 is just a storage device (like CDROM).
I mean, tricks like that are truly starting to abuse the hardware. But yeah, the 21fx actually hijacks $fffc,fffd vector reads and redirects it from the expansion port alone, so yes, it's technically possible. But it's really, really, really not supposed to be.Also SNES could do a DMA from some open-bus address to PPU, there's nothing preventing the base unit from outputting data upon PPU writes (as long as there's an I/O port for disabling that behaviour, so the CPU could also write to PPU without getting disturbed by the base unit).