No combination of holding down setup and/or reset during power-on gets me any response, so I can’t get into DFU mode.
It doesn’t show up as a USB device either:
user@host ~ % lsusb
Bus 002 Device 004: ID 03f0:231d Hewlett-Packard Broadcom 2070 Bluetooth Combo
Bus 002 Device 003: ID 04f2:b230 Chicony Electronics Co., Ltd Integrated HP HD Webcam
Bus 002 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 0424:5434 Standard Microsystems Corp. Hub
Bus 001 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
user@host ~ %
It sure looks like your device is in the “no firmware” state, but since you have a JTAG programmer, you can get out of it. You need to flash the bootloader to the device over JTAG–I would contact Particle support and ask for instructions since they may want to send you a special bin file.
Thanks to Harrison from the support team I’m back online.
Summary of the steps to fix, for anyone else who gets into this state:
Connect ST-Link according to the pinout here: http://imgur.com/1Iw202n (I saw some debate elsewhere about the reset pin connection to D3 or RST)
With the photon powered from the ST-Link’s 3v3, the D7 LED lit up. At this point I was able to enter DFU mode (edit: hold SETUP and press RESET repeatedly until the RGB led starts the mode-selection flashes )