This idea has been fomenting in my head all day, and the more I think about it, the more I like it. I haven't quite yet figured out where he fits best (AA or AAA, or lower), but I'm fairly close to convinced he would be a great hire for the A's.
There are lots of reasons why this idea doesn't make sense. For one, we're probably going to have to compete for him, and the A's are famously unwilling to compete for coaching resources. For another, Don Mattingly's remarkable offensive career was anything but an approach which represented the A's offensive philosophies. Despite a .307 career average, Mattingly only managed a .358 OBP, and was an extremely aggressive (if impossible to strike out) hitter.
That said, Mattingly also brings something to the A's offensive approach that they lack. He was an an extremely effective hitter with runners in scoring position. He was a fantastic defensive player as well, despite not being a guy you'd look at and call "athletic." And, while I can only support this claim anecdotally, he was one of the smartest hitters I've ever had the pleasure to watch.
I have no real information about how valuable the Yankee players found his instruction the past couple of years, which is a significant defecit. But if that information is positive, I'd tend to believe in a big way, based on the way he played the game.
Mattingly, I can only assume, wants managerial experience if he wants to manage in the majors, and we can offer that to him. Why not swoop in now, and scoop him up to manage one of our minor-league teams?