Gluster with s3ql mounted brick?

I am trying to test run Gluster with a volume consisting of two local bricks, the first being a plain directory and the second being a s3ql based mount.

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.

Any hints?