Work around solution:
1) Install a minimal Linux system on a small logical volume.
2) Make select this system in BIOS boot options
3) The new grub2 installation found the existing system and offered it as a boot option which I then selected.
4) I then ran the following command returning the system to its prior state (without having to resort to backups):
pvmove -n rootpartition /dev/arraydevice
This moved any logical volume extents associated with the root partition to anywhere else in my lvm pool. I then rebooted the system to confirm the fix. I'll probably leave the backup installation as a failsafe in the event of some other slick move.
Now the fact that this worked (steps 1-3) suggests that the system could be configured to boot from the raid pv, but I didn't have time to find the cause. The original question stands in case someone knows how to navigate the pitfalls of transferring an existing Debian root into a raid pv.