I thought it was worth it to check in on this request - I replied on the Github issue:
I'm also interested in USB HID for the Gen3 devices. Adafruit is currently working on their Adafruit_TinyUSB_Arduino library, which I've tested on their Feather nRF52840 Express, and it seems to work well. Hopefully this might give the Particle team a head start.
To follow up here, I'd also be interested in adding BLE HID to the stack, now that the (much appreciated!) BLE support has been rolled out. Again, Adafruit has firmware for the nRF52840 that supports this in their Bluefruit52Lib library.
This would be very useful for me, as having access to basic HID support (over either USB or BLE) is great for early-stage prototyping efforts for Interaction Designers. Keyboard emulation, in particular, is an excellent way to get a basic interaction working that can quickly migrate from a serial monitor, to a web browser, to a Processing sketch, to a Hololens (or many other paths). I currently have a Feather nRF52840 Express connected over I2C to my Argon to support this - it works, but it's quite redundant.
Thanks for the support, and for considering these feature requests!