I think the Apple silicon devices are going to have a pretty locked boot loader

Couldn't be further from the truth. The OS is heavily locked down to prevent malware from modifying the kernel / boot process, however bypassing it is as simple as holding down the power button until you see an options screen (equivalent to BIOS on a PC) and one of the options is a tool to adjust boot security including the option to boot into an arbitrary third party kernel. As long as it's compiled for ARM64 (which is a decades old industry standard CPU architecture) it will boot.

The only real headaches are around drivers. For example Mac laptop trackpads don't have any buttons at all. Instead the trackpad is pressure sensitive and the software should detect pressure that looks like a press action, treat that as a click, and send haptic feedback (vibrating the trackpad). None of that is standard stuff and if you want a Mac laptop to work at all... you need to figure it out yourself.