After finally getting my Input Club K-Type keyboard the other day from Massdrop I've been playing with getting a solid custom firmware/mappings setup with some good success. I haven't really done anything all that crazy outside of updating the firmware to a Mac keyboard layout (swapping the CMD and ALT keys) along with mapping CAPSLOCK to HYPER (LCTRL + LALT + LCMD + LSHIFT).
Below you will find the configuration JSON to use as a jumping off point. All that you should need to do is copy/paste this JSON content from the PasteBin link below into the online or client Configurator application and then simply flash the firmware using the dfu-util utility.
Input Club Configurator (online): https://input.club/configurator-k-type/
Input Club Configurator (client-side application): https://github.com/kiibohd/configurator/releases/tag/v0.1.0
Download Config: ktype-d3vnull.json (PasteBin)