Custom module menu path renaming/redirection
You could modify $_GET['q'] during hook_boot(), something like the following.

function mymodule_boot() {
  $_GET['q'] = implode('.', array_slice(explode('.',$_SERVER['HTTP_HOST']), 0, -2));
}


There is probably a regex that could do the same as that explode/implode mess, but that does the trick to start with.