Why are you asking a wheelmaker/cart tech support about a frisbee?
You don't ask a plumber to fix your window, do you?
The point, and there is one, is this: WordPress MultiSite, by design, is meant to be separate, non-related, sites. That many people want to use it differently means that as it grows it will probably adapt. But right now, we've only had it in full blow WordPress support since 3.0, so it's a baby. Give it time.
Once I create and name a menu for a site in WP MS, the admin should have the power to use that same menu for another site on the same machine.
The major flaw with this concept that I can see is not every site uses the same themes, so a menu that works perfectly on site A may break on Site H.
Not to mention the hassle of sorting out how to copy it over, how often do you sync, how do you handle permissions?
Right now, not easily possible. Maybe someone can come up with a plugin for it. :)