Pluggo 3.1.1 is now available for OSX.
System requirements call for a G4. I've experimented a bit, running 1 or 2 plug-ins. Seems to work fine with Metro on my G3 350MHz.
Info page is here:
http://www.cycling74.com/products/pluggo.html
Demo can be downloaded from this page:
http://synthesisters.com/pluggo3/wizard31-1.html