Details on Ubuntu's UEFI secure boot plan

You probably know more than I do about it but I don't think the GPLv3 demands the release of private key material, it is only concerned with the owners practical ability to load their own modified software on the device, which is far more easily serviced by providing a mechanism to manage keys local or to disable signature checking on boot. In any case I'm sure someone could crate a contrived example where the only way to comply with the GPLv3 is to disclose signing keys but I don't think that is how it works in practice.