Ok, so I got my stuff.
I added the Zcover clip to the Otterbox.
There are a few parts to the clip set.

There is a back tab that goes inside the case of the device (shown here the one from my old Zcover case for my iAudio X5):

I installed this by drilling 2 holes in the appropriate places in the Otterbox out case. I installed it between the outer case and silicon inner case.

Next there is the clip that goes on the outside of the case and locks onto the tab:

This is the universal connector used to connect to various Zcover clips. This fits just right with the tab installed between the 2 layers of the Otterbox.

Next is the quick release belt clip:

The connector clip slides and locks into this, and can rotate once locked in. There is a button on the belt clip to release the latch so the device can be pulled out. Mine is a bit tough to unlock/lock at times, but I suspect it will loosen up a bit with time.

All together:


The installation of the clip seems to have minimal interference with the Otterbox installation. You'll notice I have a decoration on my N900, I pulled the cord to the side and have it feeding out the edge of the USB cover, which doesn't seem to have interfered.
I did notice that 2 of the clips of the Otterbox on the bottom of the keyboard don't seem to have gone up quite as far as they should (I assume they are supposed to click over the edge of the keyboard like the corner clips). I assume this is from the slight difference in overall height due to the zcover tab behind the phone. However the Otterbox is still very secure and tough to remove.


The full image gallery is here.

Overall thoughts:
It was fairly easy to mod the Otterbox to install the Zcover clip.
The clip does not seem to have affected the installation of the Otterbox in a significant way.

Benefits:
You get a quick release belt clip along with your Otterbox (or whatever other Zcover clip/holder you want to use).
You get a loop on top of the clip on the phone that you could attack something to.

Drawbacks:
You can't lay your N900 flat with the clip.
You can't stand your N900 straight on the kickstand with the clip because it changes the balance and causes the N900 to lean a bit to the right.
The clip adds a bit more bulk to the N900 (and to your belt, but any belt holder would do that).
The extra depth from the tab in the case prevents a "perfect" installation of the Otterbox on the phone, but this does not seem to affect the stability of the install.

Other:
I tried a couple times to get the Otterbox screen protector on and then gave up on it.
I think my room is just too dusty to get something sticky like that in place and properly sealed.
I guess I'll have to look for other screen protectors that might be easier to install, and try to install in a cleaner environment.

Tomorrow I'll be out and around a bit, so I'll see how the phone feels attached to my belt.