KRIKzz, the creator of the Everdrive products for retro consoles, has announced that he is working on a new project for the Nintendo 64. Not many details are known so far, but he plans on using SD for flashram (unless it is too slow, in which case he will be using CompactFlash (CF) card memory).
Check the following link below for photos of his current development. You can see the device is plugged into an open N64 console and wired to an Altera logic development board.