Great idea - this (I think) allows me to do just about what ever I need to do.
I've quickly checked it out by creating a second Structure and also categorised it so that the 'lower' Groups cannot inadvertently toggle into it from the top right Structure menu. I then created a new top-of-structure page for the more advanced menu and copied all the other existing pages into the new Structure.
Only one small downside which is that the higher Group can in my case toggle into the 'lesser' menu - quite often a page drops out of Structure mode ie sometimes after an edit and so on - the top right select menu then gives my 'higher' Group a choice of the two Structures since in my arrangement the higher Group is in both the higher and lower Categories.
Thanks again - I think this has cracked it for me.