Having just the plain directory as a unique brick works fine.
Having just the s3ql mount as a brick does not work:
drwxr-xr-x. 4 root root 4096 May 13 19:57 . dr-xr-xr-x. 22 root root 4096 May 13 18:25 .. d?????????? ? ? ? ? ? gluster_testdir
Having both bricks added only the space of the first brick is showed as available, plus nothing can be written or read from the volume.
I am not sure if it does not work because Gluster is trying to re-mount (mount an existing non-local mount) the s3ql mount or because Gluster does not see the s3ql mount as a "real" directory even though it should.