Loading custom CA keys into the UEFI store doesn't void your warranty and is available as a standard menu option on every UEFI setup screen I've seen. The key store can usually also be reset to accept Microsoft's keys again for when you want to run Windows.
As long as you know the password to your UEFI firmware setup, you can enroll keys or reset the key store. Of course this process is way more complicated than necessary (especially in Arch's documentation), but there's nothing preventing vendors like Canonical or Fedora from pre-signing kernels and drivers.
If you want to use your/your vendor's keys for existing drivers, you/the vendor can also have sign those files.
The only difficult step for the end user is enrolling the MOK key, which requires selecting the right option in the UEFI setup and a reboot or two. Anyone can set up a repository of presigned keys and kernels you can rely on, whether that's you or your favourite open source project.
The problem is a lack of availability because the people who would need this, open source enthusiasts running Linux or *BSD, often just disable secure boot entirely. Nobody has made a nice GUI for managing this stuff either, because everyone who messes with this stuff knows the command line anyway.