The twist here, is that you should install these libs to a separate location from the regular system libraries. Since we're compiling PPC 10.2 specific versions, we don't want to overwrite our regular libraries. I went with /opt/1028libs/. Just set this with the --prefix option to the ./configure script, for all the libraries you're including. Do this, and make sure you've defined the above environmental libraries both when you do ./configure and when you do make, and you should be set. For some of the libraries you might also need to use the --host=powerpc-apple-darwin option when running the ./configure script.
Once you've set up all the dependency libraries, you're ready to build ScummVM itself.