It’s not something that would probably get reported widely, but the bputil command (which manages boot policy, as the name suggests) and kmutil now work as of the second beta of 11.2 and allow for you to provide your own code to run.

This is probably what was being referenced: https://news.ycombinator.com/item?id=25772462

"macOS Big Sur 11.2 beta 2 is out with full custom kernel support" ... "The OS now finally includes the firmware and bootloaders and tools necessary to replace Big Sur with not-Big-Sur. That was previously not possible."