I was able to create a project in Phabricator called "ζ Architecture" using the Z Greek letter and it works correctly.
Then I was able to upload/commit/push to git a file with a similar name (i.e. using the "ζ" letter), but when I click on the web link provided by Diffusion (somethin like: https://phabricator.foo.com/diffusion/ZARCH/browse/master/The%20%CE%B6%20Architecture%20v2.0.txt) I get the following error:
// **Path Does Not Exist**
This path does not exist anywhere.