If you really need a moderator of moderators, just have the moderators nominate someone other than themselves. I think you'll have a better time than bringing someone new on straight to a higher position. You can also always bring in another regular moderator to replace their previous role.
Or you could allow majority vetoes if there is a dispute.
Are there often issues that come up? I don't remember seeing much in the past, but I know it's been a while and with site rule changes, if may be necessary.