All I know is that you can't get antialiasing on ZDepth - full stop. Which kind of screws up it's compositing capabilities.
I looked at this a while back, got frustrated and then left it.

I don't know anything about Fusion but there is a 'trick' you perhaps already know if you want to get an antialiased ZBuffer.
First create a surface shader material.
Then Create a Sampler Info utility(That stuff is definitely everywhere!)and a Set Range Utility.
So now if you don't change the names you should have a 'Sampler Info' called samplerInfo1 (it is case sensitive so beware!) and a 'Set Range' called setRange1.
Connect the outValueX of your setRange1 in your surface shader outColorR, outColorG and outColorB attributes.
Type 1 in the Min X attribute of setRange1.
Leave 0 in the Max X attribute.
Now let's create the following expression in the Value X of setRange1 :

setRange1.valueX=mag(<< samplerInfo1.pointCameraX,samplerInfo1.pointCameraY,samplerInfo1.pointCameraZ >>);

Call this expression whatever you want.

Et Voila !

Now you can use this shader to render an antialiased ZBuffer
you set the 'near' (it is white in the rendering) with Old Min X in setRange1 (type 10 for example) and the 'far' (it is black in the rendering) with Old Max X (75 for example).

This is very useful I think.