After failing miserably to recover one of these a couple of years ago with a bootloader and following the directions here I tossed it in a drawer and forgot about it until I bricked another one the other day (my fault, forgot to change the board selection before uploading)
I took another stab at it, this time following the directions to use another Arduino (MKR Zero) as the programmer.
It worked, but with the caveat that in Step 10, when using the MKR Zero, the define must be set to 28, ie:
#define SD_CS 28
Also, even though the MKR Zero has a card slot and despite the instructions, do not change the line:
if (!SD.begin(SD_CS)) {
or it can't find the card.
Finally, while the above link instructs one to connect pins 9, 10, and 11 for SWCLK, SWDIO and RST respectively, the required flash_from_SD example sketch from the Adafruit library defined SWCLK as pin 11, SWDIO as pin 12 and SWRST as pin 9. I edited the sketch to conform to the instructions above and it worked fine.