And I forgot to mention earlier about saving on this cart, it is intended that PRG-ROM is used for save data. Battery-backed SRAM may be faster to write, but the bad part is how easily the data gets corrupted, without using a real NVRAM controller. I've only found one cheap IC that is simply an NVRAM controller, but it's long out of production - most newer ICs that handle it cost too much because they include a lot of other features (like real time clock, CPU reset generator, watchdog timer). First time I made it to Chaos in Final Fantasy, my dog's tail hit the NES which erased my save (real life watchdog reset?), I think using FlashROM can put a stop to crap like that. Just don't bump it WHILE it's saving, but in practice you would probably want to keep 2 copies in memory, to revert to a backup if the first has failed.
Since the sector size for the Flash is 4kB, that means you can program individual bytes in whatever order you want (you could leave $FF "blank" bytes in your data and fill them in later, for example), but when you erase, it is done in 4kB-sized blocks. On the original Squeedo design it was 64kB sectors, that was a little inconvenient for NES, heheh. But, in theory a game saving to flash could be 128kB, and have 384kB of saved data! Who knows what for, but it may be a good way to write a live level editor on the NES or other stuff.
Since the flash endurance question usually comes up too, here is the calculation for that - 100k erase cycles - a player could save 10 times a day, every day, for 27 years straight. Use 2 sectors for saving and it's 54 years, and so on.
Good to hear about the famicom thingy. I forgot about the attached controllers so you cannot use it for debugging (except with an AV famicom) and the expansion port is different.
Yeah Famicom exp port is really close to be being a normal DA15 connector, but I don't think they can be bought without the metal flange. At least it's not something totally alien like the NES exp port.