Try VNCThing (I use it myself). It is Carbonized, but works fine in OS 9 with the following constraint: NEVER switch to another app while you're using it; make sure you close the VNC window first. If you don't it seems to confuse the VNC server, which is a bad thing -- it does not handle things well when it's in a preemptive environment like OS 9, so always keep it in the foreground until you close the window.
What I do is keep the app running, and open and close windows as I need to jump in and out of it. It makes up for this very annoying limitation by being the fastest I've used on OS 9, and being able to scale screen size (my G4 runs at 1024x768 normally, but the G5 is at 1280x960, so VNCThing scales it down). While cranking out Classilla code, I'll pop in and out of my G5 workstation without having to flip the KVM all the time.