Thanks for the update... I wonder if that's a sign they are going to be releasing their boards soon.
Zip up Adafruit's SAMD51 code and write a JSON file, to create a new entry in the Arduino IDE boards manager, so that the new board and code could be installed.
You don't HAVE to have a JSON file. You can still copy new directories into the hardware/ tree directly. I've got a 1.8.2 IDE with espressif ESP32 and SAMD11 boards added that way...
The SERCOM peripherals' pads are located on the same pins on both the SAMD21 and SAMD51 devices. On the SAMD21 it's possible to mix 'n' match practically any SERCOM signal to any pad. On the SAMD51 however, while the RX/MISO signal can be assigned to any pad, the USART's TX output is limited to the pin at pad 0