From the user app on the Argon/Boron, is there a code snippet that can induce safe mode, or can it be done via USB from the device itself?
By USB, as long as the device is running 0.9.0 or later and you have an up-to-date Particle CLI:
particle usb safe-mode
By USB serial, connecting at 28800 baud will go into listening mode, then sending the L (uppercase) command by serial will cause it to go into safe listening mode. It will look like listening mode (blinking dark blue), but it’s running safe mode underneath it.