I've changed porting layer, mainly because of usage of GHS instead of GCC and board definitions file to adapt to MCU and board specifics. You are not obliged to use GHS and you'll have less work to do if you use GCC-like compiler, because GCC is the original compiler, used by ChibiOS. Porting is quite fast, for OS you mainly have to change context switch code and add PIT for the OS tick. In order to run test/benchmarking you have to develop also serial port driver to take data out of MCU.
Maybe a week should be sufficient as time and then it comes to testing. ChibiOS code is quite good, for over a year I didn't found major problem in the OS kernel, even if I'm evaluating very old 2.6 release.
There is no public release of the port, since not many people are using GHS and I have no time to do the same for GCC. At the time when I was playing, GCC was not officially released yet for MPC5748G.