The remote branch from git-svn is pretty much the same as a regular git remote. So in your local repository you can have your git-svn clone and push changes out to github. Git doesn't care. If you create your git-svn clone and push the exact same changes out to github, you'll have an unofficial mirror of the google code repository. The rest is vanilla git.

git svn clone http://example.googlecode.com/svn -sgit remote add origin git@github.com:example/example.gitgit push origin master

Now that you have this, occasionally you will have to sync the svn repo with git. It'll look something like:

git svn rebasegit push