Here’s the guts from [Dext0rb's] Super Nintendo cartridge. It’s easy to pick out the dark-colored board which lets him reflash SNES ROMs via USB. We’ve seen this done a number of times, but this is a much cleaner option than hacks that just add a dead-bug-style memory chip.
The board he designed has a double-row of pin headers sized to fit the footprint vacated by the original ROM chip. The board has a mini-USB connector which can be accessed through a hole he cut in the side of the cartridge enclosure. This is in the right place so that you cannot plug it in when it’s being used in the SNES (which would cause damage). The ATmega32u4 chip handles USB connectivity and programs the 32 megabit flash chip which stores the ROM. He’s posted a few articles on the blog portion of his site which you’ll find interesting. We suggest starting with this hardware teaser.