For free software, I wonder if reproducible builds plus a "certificate transparency"-style check in the updater (only allow an update once several build servers, preferentially located in separate jurisdictions, have validated the build and published the corresponding source code) could help. That is, make it impossible to push a backdoor to a single user without making it public to everyone. Making updates anonymous…
And for bonus brownie points we could have reproducible build checking (a-la certificate transparency) against the source repo, to see whether the binary is different to the official one. However, I think the threat model might have to be reconsidered (if all the developers are compromised, couldn't they upload a bad hash to the certificate transparency trail with a dummy version that only one user is given?).
I might write a blog post about this actually, though I'd need better experts than myself in IMA (which is what you'd use on Linux for this) and other secure-boot work.