After another round of discussion we sadly had to decide this isn't really
bullet proof though.

Quoting Gebi from the according discussion on IRC (with his permission):
    
02:48 < gebi> you know that /tmp was to bind mounted too if you want to use X?
02:49 < gebi> not a good idea to use a bind mounted /tmp inside of the chroot
for saving state which ultimatly goes back to the base image
02:50 < gebi> mrud: but you e.g HAVE to mount --bind /tmp chroot/tmp to use X in
chroot
02:51 < gebi> so it would be no fun to use chroot/tmp as state dir for
grml-chroot, because on unmounting your files would not be there anymore
02:51 < gebi> because hidden by the mount --bind

So unless someone provides a tested and bullet proof solution we've to close
this with won't fix.