Speedup of Outdoors Culling

I am trying to find a way to speed up the rendering of the outdoors in TGE. After looking though the code, I see that TGE uses zones for culling.

An interior can have one or more zones; yet, the whole outdoors is contained in one big zone. So, when rendering outdoors the engine has to go though every object in the outdoor zone and cull each one separately.

What I would like to do, is implement some sort of culling intelligence for outdoors.

There are two options I have come up with:

Octree I saw in a forum someone did this, but it was only in passing was it mentioned. The problem I have with this is the continuous terrain of TGE. Again, two ways of getting around this. 1) Use non repeating terrain (There is a resource about this) and apply an Octree. 2) Use the mission area to apply the Octree. As far as I can see this would be a major undertaking for changing the zones.

Placeable Zones This would be a zone manager that can contain objects, and other zone managers. This way, you can control what gets culled easier then with an Octree. Again, the current zone handling will need changing.

In my estimate, the Placeable Zones would be the lesser of the two evils.

Does anyone have any other ideas?

Has anyone done anything like this before?

(BTW, I'm using TGE not TSE. So answers like it's in TSE will not be appreciated. And answers that GG is working on it is nice, but we are waiting for a lot of things from them and I need to get this working now.)