Well that's the real trick. It would be difficult to access the menu bar if you had a mess of weirdly placed and sized windows scattered around, like the OSX UI paradigm assumes you do.
But if you have a tiling window manager, and especially if some of those tiles are full height, it's quite easy. Just put your mouse in the window you need, and then go straight up to the menu bar.
Yes, once in a while you need to cross some extra real estate with the mouse in order to "dodge" a UI element you don't want to switch focus to, but it's really no problem.
I've been using tiling UI and focus-follows-mouse on snow leopard since 2009. I couldn't work any other way.
The global menu bar is indeed a problem, but it can be alleviated by adding a small delay before changing focus (so fast mouse movements to reach the menu won't change focus).
"Mac OS X interface and ergonomy is (as said by apple) not 'compatible' with the focus-follow mouse mode. Why ? Because a single application interface is split into lots of different elements. For exemple, you use an application like photoshop or word, you'll have different panels floating around the main window. With sloppy focus mode enabled, with an application below, you would'nt be able to reach them. And how should they consider the desktop, which is, finally, just a 'root' window managed by the Finder..."[1]
Here are some more.
"Sadly, it seems that for the time being there is no good way to do this in OS X."[2]
"The fundamental problem with sloppy focus on the Mac is that the menu bar is always associated with the currently focused application; if you had sloppy focus, accessing the menu bar for a specific application would be supremely difficult."[2]
[1] - http://jeremy.zawodny.com/blog/archives/000149.html#comment-...
[2] - http://superuser.com/questions/27306/focus-follows-mouse-or-...