The correct thing to do is similar to SPARC in that you create a 100Hz interrupt (possibly using the PPC decrementer?) that should increment a variable representing the number of ms elapsed. All you need to do is then point the new code towards that variable and it will handle get-msecs and milliseconds automatically.
The interrupt routine is probably do-able in about 20 lines or less of PPC asm but it's beyond my current PPC-fu. It would be a good project for you though, and would probably fix the incorrect timing CIF milliseconds with -M mac99 :)
There is no reason at all to use the decrementer and interrupts, with all the problems inherent in that; just use the timebase, it's what it's for. The decrementer and timebase tick at the same frequency.
Segher