I managed to get Kiwix to finally work on OS X 10.5 (PowerPC) even though I don't think they ever had a PowerPC-working build. There are some PowerPC builds around but the components inside the app bundle (contentManager.dylib, xapianAccessor.dylib, zimAccessor.dylib and zimXapianIndexer.dylib) were all build for Intel-only. Therefore, Kiwix did not work.
It does work now after re-building the components and the XUL framework, and I can browse Wikipedia offline from the safety of my iBook G4.
If you enable ZIM indexing, the CPU will surely get gimped for few hours, as seen in my screenshot.