Nuked eMMC on Beaglebone Black, now can't boot from SD

So apparently I did something stupid. I was booting Kali Linux from an SD card on my Beaglebone Black rev. B, and I got it in my head that it’d be a good idea to wipe the eMMC entirely and make it a swap partition. I figured if I rendered the board unbootable that I could either force it to boot from the SD card by holding the boot button or I could connect it to a computer with a USB cable and manually re-flash the eMMC. Well, it did become unbootable, except the boot button does absolutely nothing and the eMMC isn’t presented as a block device any more. I have an FTDI cable and watching the serial output at boot I no longer see the usual stuff. The Beaglebone is now only sending the letter C 8 times. Connecting the USB cable to a computer creates the usual ethernet interface and I can see DHCP packets being sent from the Beaglebone as if it’s attempted to bootstrap itself from the USB networking, but that’s all that happens. No amount of Googling has turned up any answers - everything I’ve read about unbricking assumes you can at least boot off the SD card. There are some references on this page to some tools that sound like they might help, but the links are mostly broken. Does anyone have any suggestions as to how to resurrect this thing or should I just pitch it in the trash?