Would it randomly pick one of the bootable disks? That way you have an n-1/n chance of avoiding the bad disk?
If a disk with a boot vol failed, it would have a 50/50 chance of still booting depending on which one failed. You'd create a backup grub entry to boot off the other one manually.
Not as transparent as hardware RAID, and maybe now grub is aware enough to auto handle mdadm? I don't know - I haven't done bare metal mdadm for a decade or so.
The same way it has worked for the past 30+ years: the firmware boots from first designated boot device; if that device isn't bootable, it moves on to the next one. The next one boots since it's part of a mirror and has all the necessary data to do so, and in the rare case where the device is "half bootable" one would simply intervene and select the next good bootable device manually.