This is a (4) disk array, running mdadm's raid10, and was created with the command:
$ sudo mdadm --create /dev/md100 --raid-devices=3 --spare-devices=1 --level=raid10 /dev/sd[bcde]1
After creating the array, I can check the details with:
$ sudo mdadm --detail --scan /dev/md100
ARRAY /dev/md127 metadata=1.2 spares=1 name=freya:100 UUID=deafbeef:deadbeef:beafdeff:beaffffa
Notice the "name=freya:100". That's the key to finding a static path to the array. If I then look under the /dev/md directory, I will see:
$ ls -l /dev/md
lrwxrwxrwx 1 root root 8 Dec 30 08:15 freya:100 -> ../md127
That means the static path to this array is "/dev/md/freya\:100" and I can use LUKS format on it with:
$ sudo cryptsetup -y -v luksFormat /dev/md/freya\:100
Alternate, I can search for the UUID in the /dev directory and find:
$ sudo find /dev -name '*deafbeef*'
I can then add a LUKS keyfile to the device and unlock the device at boot by listing it in the /etc/crypttab file. Either path will work, but the colons (:) will likely have to be escaped in /etc/crypttab.
PS: Yes I've tried putting the array line in the /etc/mdadm/mdadm.conf file as just "ARRAY /dev/md100 UUID=deafbeef:deadbeef:beafdeff:beaffffa", which is supposed to fix the issue.
PS #2: It's interesting that the mdadm UUID appears in /dev/disk/by-id and not /dev/disk/by-uuid.