Hello therebelrobot,
I am pretty confident that the msd method will work with the XIAO BLE.
When pressing the (tiny) reset button on the board twice with the board connected to a computer, the flash partition shows up on the computer as USB storage with the name “XIAO-SENSE” (used to be “Arduino” for SAMD boards, as you mention).
See the documentation for flashing with CircuitPython firmware.
My guess is that the firmware file name does not matter so much, and any *.uf2 file would then be accepted.
Good luck with your experiments.