The firmware's already open source, and you can reflash it by either opening the device to remove a write-protect screw (for older models) or with a debug cable

The firmware's already open source, and you can reflash it by either opening the device to remove a write-protect screw (for older models) or with a debug cable. https://mrchromebox.tech/ https://mrchromebox.tech/ has UEFI firmware for many models, and many also support BIOS boot without needing to remove a write-protect screw or use a debug cable. Also, all support running custom Chromium OS builds via the usual ChromeOS boot mechanism.