David,
It is not intentional for volatility.plugins.overlays.mac to be missing from setup.py (it was probably missed when merging the old mac branch into trunk). However, unless you plan on using volatility as a library (i.e. importing it from other Python scripts), you don't need setup.py at all.
$ cd volatility
$ cp <PATH TO YOUR PROFILE>/Mac10.6.zip volatility/plugins/overlays/mac
$ python vol.py --info | grep Mac
Before the 2.3 release, setup.py will be fixed in case you do plan on installing volatility as a library. Also, pre-built Mac profiles for all common OS X kernels will be available at that time, so you won't need to build your own.
MHL