If I had the time, I would use the RP2040 version and program it to act as a SPI device obeying a subset of the SD card protocol. Then the SPI browser would work out of the box.
As for writing to the floppy, I always meant to add an OS layer and writing support to the SPI SD interface. The idea is to locate the OS code in bank3 of a 128k gigatron. Here are a couple ideas for an os interface at https://github.com/lb3361/gigatron-os/b ... /oscall.md. I never had a long enough piece of time to do this seriously. Then one has to modify TinyBasic and MSBASIC to save/load using this interface. Any takers?
It seems that there is a promising path using the Adafruit floppy interface https://www.adafruit.com/product/5679 and a Raspberry Pico because they also provide a nice controller library https://github.com/adafruit/Adafruit_Floppy/ with read support (write support seems to be under development). One could then write a raspberry pico program that takes SD-card like commands on a SPI port and executes them on the floppy (one of their example takes USB commands instead of SD-card SPI commands, that's close!)