Folks, I don't see where the problem is? All the installer does, is move the files from the CD to the HDD. cfg-Files etc are generated in ~/Library/App.-Support automatically if I remember correctly.
So, just grab the quake3 folder from the CD, copy it to your HDD and replace the original Q3 application with the OS X version that can be found at MacGameFiles or Versiontracker for example. Et voila, done it a dozen times, always worked like a charm
EDIT: I'm not sure, but from what I understand there, an UB update available for Q3A, should not be a problem at all then.
http://www.versiontracker.com/dyn/moreinfo/macosx/8961