A DMA transfer to CHR-RAM would be great. In theory, it might even be possible on a real NES, if $2007 can handle writes that quickly. (use a bus conflict to force sprite DMA to write to $2007). Or simply dual-port CHR-RAM.

Or something like the Color Dreams' Hellraiser cart, it had it's own CPU running on the CHR bus.

This stuff isn't impossible on NES, just sounds like an unlikely cartridge. : )