Is there any way to reach the built-in FDS hardware while a cart is inserted? If so, imagine what sort of disk-to-disk c0py1n9 shenanigans you can do.
Even assuming you run off the 2KB of stock RAM, you'd need to modify the hardware to make it even remotely bearable as you'd need to constantly switch between disk and cartslot (remember both would be trying to fight over the same address space). A modded Twin Famicom like that would make for a good disk copier though...
How did the kiosks work? I know those were based off FDS, no idea if they have extra RAM or just require you to switch disks often (by doing a partial copy each time)
I thought the Twin had a socket on the right side with a cover, intending to allow you to plug in an FDS unit without an extra RAM adapter.
No, that's just the same sliding I/O port that the FDS RAM adaptor has on it.