"Anyway, I have a horrible suspicion these will have locked down UEFI firmware and locked bootloaders"
I don't think it likely. Firstly most of these wont be made by Microsoft so it will be an OEM decision. Secondly near zero people would install an non MS OS and if it's Intel hardware both manufacturers and users would probably want the option of installing other Windows OSs too.
And whilst we know Windows 10 has historically had far fewer holes in boot loaders, etc. than Android / Linux, etc. to compromise such a system, someone normally always finds a way if there is a demand!