On reviewing the setup obsessively, I realized that a locking issue will prevent smooth failover for maps in the event of an NFS server failover. As mentioned in T203469, NFS failover is not very good without shared cluster filesystems, so this isn't the only place where that is true. Currently I will be documenting the failover as suggesting that the maps servers be rebooted to clean up after.
Other ideas are possible (active/active DRBD volume with another data migration can be done with the spare disk on the server).