Ah nostalgia. I started in the games industry doing N64 programming, and our devkits looked similar to those flash carts but there was a big SCSI cable connector that went to the host PC.
Later on we had some "debug" cartridges that sat in between the console and a flash cartridge and intercepted reads/writes to a single address and replaced the data with whatever came in over a serial port back to the PC. That was enough of a backdoor to implement very basic dynamic reloading of art assets - I wrote a Photoshop plugin that added a "send to N64" button and some game-side support code and the cycle time for artists trying to see their work on a TV went from 30 minutes to 10 seconds.
aappleby · · focus · HN ↗
Later on we had some "debug" cartridges that sat in between the console and a flash cartridge and intercepted reads/writes to a single address and replaced the data with whatever came in over a serial port back to the PC. That was enough of a backdoor to implement very basic dynamic reloading of art assets - I wrote a Photoshop plugin that added a "send to N64" button and some game-side support code and the cycle time for artists trying to see their work on a TV went from 30 minutes to 10 seconds.