Actually this would work just fine with Gluster. Except in the case of gluster-NFS, the native client is only acting like a router/proxy of syscalls to the backend system. A directory opened by an application will have a matching directory fd opened on ext4, and readdir from an app will be translated into readdir on the matching fd on ext4. So the app-on-glusterfs and glusterfsd-on-ext4 are essentially "moving in tandem". As long as the offs^H^H^H^H cookies do not overflow in the transformation, Gluster would not have a problem.
However Gluster-NFS (and NFS in general, too) will break, as we opendir/closedir potentially on every request.