Dear Chris,
... I'd like to add a custom exception for https://www.kicad.org/libraries/license/ which is an exception to the Creative Commons CC-BY-SA 4.0 License, but is not registered in https://spdx.org/licenses/exceptions-index.html ...
Hello! SPDX working group member here - that looks like a good candidate for a new exception request for the SPDX License List. There's an online form that you can use to request a new license or exception:
https://tools.spdx.org/app/submit_new_license/
I'd also be more than happy to make the request on your behalf if you'd like! If it's also approved by the other SPDX Legal Team members, you'll be able to use it with REUSE in a couple of months time :)
Best wishes,
Sebastian