Hi Cam,
To my understanding if you want to provide a global navigation to user, transalates to giving content access to users from one portal into another.
My suggestion is that you look into the DNNMasters Multi Portal User Sharing 2.0, here is hte link to their site http://mms.dnnmasters.com
I am not a sales person for them :) but I think its will be a good fit for your requirements as it unifies user management of several portals so a user from one portal can transparently navigate to another without any knowledge of the transfer.
Now as far as the Navigation is concerned, you can create that using the defalut DNN Navigation Providers, you have several options here so please read thru the documentation that came with DNN (DotNetNuke Navigation WebControls.pdf).
You can have multiple instances of menus in a single skin, so for the skin of each portal you can have one navigation for portal content itself and another for navigating to other portals, now when a user logs in he can use content on the same portal and when he navigates to another portal he doesn't have to log in again and has access to all content on all other portals too...
This will make user management much easier and user experience between portal transparent and less cumbersome.
Hope this helps