You probably want this old "universal" .app client. It still works perfectly with OSRS servers on a 10.6 intel machine, and possibly on 10.5 intel, but not on powerpc. I'm no longer interested in java on powermacs, but if you can get a newer java version by any means this *might* just work as the errors that get logged seem to report the java VM on powerpc is simply too old to run it (my intel 10.6 machine has java 6)
I remember playing runescape with a 466mhz powermac G4 in 2002. I doubt you'll have any luck with runelite or osbuddy.
Good luck!