Can my web app use a UNIX socket on OS X Server?
It seems that the answer is No as OS X Server (3.2.1 as of today) comes with Apache 2.2 and from the Apache mod_proxy documents it seems that UDS support wasn't implemented in that release. I was reading a later version's documents.

Not wanting to recompile Apache on our production server, I will have to content myself with TCP ports - no real problem.