Structure based permissions are coming in the 1.9 release along with search permission sensative results. These are both being worked on now in 1.9.
We should have a DR4 (Developer Release) ready by the end of the week, but it certainly wont be classed as stable until at least Dec or Jan.
I use a similar sort of setup on damosoft.net for users documentation, based on structures, Im actually using a 1.9 there, but Im confident I can fix any problem I come up with. If your in that position, then please help us make 1.9 super stable.