I’m here one year later, but still someone might find this usefull, as I’d be damned if there weren’t dozens of ppl having this problem.
Here I have solution which works 100% …well, it worked for me
Mine started creaking after around 1000 km. I pretty much tried everything as the OP with no avail.
Then I found the service manual: https://www.sram.com/globalassets/document-hierarchy/service-manuals/zipp/wheels-and-hubs/zipp-cognition-v2-hubs-service-manual.pdf
..where on the page 4 it mentions this:
And as I had nothing to loose, I gave it a shot andd performed the “Every 100 hours” service.
The only kinda problem was obtaining the Zipp Cognition Oil as on the most of the websites it was out of stock or on backorder, etc.
Eventually I got one and performed the hub clutch removal/installation.
It’s very easy procedure and most of the users can do this at home, trust me.
When I took it apart I was surprised by how little or almost no oil was in the clutch compartment.
During installation, the service manual says "Use a small syringe to apply approximately 1 mL of Zipp Cognition oil … " …so here I’d like to put emphasis on the 1 mL which IMHO looked not enough. So I gave it like 1,5 almost 2 mL. And yes trust me, it’s fine, there is enough of room to take it
Anyway, so I assembled it together and here comes the celebration part, the creaking was gone 
BTW #1: Perhaps any oil would work, but I didn’t want to risk it especially since I was able to obtain the original Zipp oil fairly easily.
BTW #2: Still I’m unsure which part of hub clutch was making the creaking noise, but I’m roughly betting on that pink foam springy ring not being oiled enough and rubbing over the clutch. But that’s just my guess..