I think this design still works if you only allow generating a signing keypair if Activation Lock is disabled? I think that still satisfies the requirements of GPLv3 sect. 6, although we're probably in territory extremely far from settled caselaw.

> “Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made.

The "method"/"procedure" is signing out of iCloud, doing a factory reset, and holding down the button. It doesn't interfere with the continued functioning of the modified object code, i.e., of the OS itself, just with any user data present before you started the procedure.