Shared spares are already supported and have been for a long time.
You need to the mark the /etc/mdadm.conf entry with something like "spare-group=global" (you choose the name, there can be a number of independant spare-groups) and you need to have "mdadm --monitor" running. When a drive fails, mdadm will see if another array in the same spare-group has a spare, and will move it across if appropriate.
Read the man-page again.