Yes, I was thinking about a "ROM" disc several times. So you could use some of the 16K roms in these modern rewriteable ROM expansions as a read/write mass storage device.
On the other hand a 3,5" disc can store 720KB using the FAT12 format, so you can put most of the stuff there. And because of the multitasking/multiprogramming environment, you just load it once and then keep it in memory. Hm...