According to @bmike: "You can relax the boot security and bring any OS you please to the hardware - all this is documented in the Apple Platform Security guide and this KB, @WHO'sNoToOldRx4CovidIsMurder"

(Putting this answer here as, even though it doesn't work for me, it's the documented answer.)

According to @bmike: "You can relax the boot security and bring any OS you please to the hardware - all this is documented in the Apple Platform Security guide and this KB, @WHO'sNoToOldRx4CovidIsMurder"

(Putting this answer here as, even though it doesn't work for me, even with Genius Bar help, it's the documented answer.)