My point is the signature can be combined after the fact. Basically, I can give you a binary that is the signature for another binary that is the boot loader, built to certain specifications. You build that, and combine with this signature, and you are good to go. If the build doesn't build exactly the same, signature will be invalid and you can't use it.

Note that this is VERY limited in how much it could possibly help. But I don't see why it wouldn't be permissible by all licenses involved. It doesn't require the signing key, but a valid signature. For that device, it is locked down enough that the only installer that can work is the one that matches this signature. If installing somewhere else, you don't need the signature anymore, and can make your own or use an unsigned loader.