I guess OSX is unixy enough under the hood that standard unix ways of installing planeshift ( essentially identical to linux, freebsd and solaris ) should work properly..
I have heard that \'fink\' lets you download and install opensource stuff, so if you install the fundamentals like the GNU development toolchain ( gcc, binutils, make, libtool, automake/conf, m4, etc ) and then the software reqirements of planeshift and friends ( cvs, curl, jam and so on ), you should be able to build from CVS sourcecode just like any other unix, in theory.
In practice this might not be so easy, but its an option that will give you a 100% up-to-date client ( or a client that is more up to date than the server, which can be a problem ) at all times.