May I ask you and everybody in GNU organization to again include Legacy Grub as a normal package in GuixSD and in everything? The reason is that it is just terrible to install the system and after not to have access to it or invent something how to boot it. Grub2 is invalide. For what anybody does need such an entity which cannot install itself on a arbitrary prtition? Of course, people use different operating systems, which are on different partitions. I always install grub on the root partition. But Grub2 does not work, because you specially cut its ability.
In Nix Grub2 does not work too, but they have Grub1, which is installable anywhere without any complications.
If you are afraid that people will blame you when they spoil their computers, you, please, write a warning, that you should use Grub2, etc.