In case someone is still searching for an answer, from the documentation of Proftpd :
Filesystem Tricks A typical scenario is one where "DefaultRoot ~" is
used to restrict users to their home directories, and where the
administrator would like to have a shared upload directory, say
/var/ftp/incoming, in each user's home directory. Symbolic links would
normally be used to provide an arrangement like this. As mentioned
above, though, when chroot(2) is used (which is what the DefaultRoot
directive does), symlinks that point outside the new root (the user's
home directory in this case) will not work. To get around this
apparent limitation, it is possible on modern operating systems to
mount directories at several locations in the filesystem.
To have an exact duplicate of the /var/ftp/incoming directory
available in /home/bob/incoming and /home/dave/incoming, use one of
Linux (as of the 2.4.0 kernel):
mount --bind /var/ftp/incoming /home/bob/incoming
mount --bind /var/ftp/incoming /home/dave/incoming
mount -o bind /var/ftp/incoming /home/bob/incoming
mount -o bind /var/ftp/incoming /home/dave/incoming
BSD (as of 4.4BSD):
mount_null /var/ftp/incoming /home/bob/incoming
mount_null /var/ftp/incoming /home/dave/incoming
mount -F lofs /var/ftp/incoming /home/bob/incoming
mount -F lofs /var/ftp/incoming /home/dave/incoming
The same technique can be used for directories, which also
operate in a chroot()ed environment. Also, it should be possible to
mount specific files this way, in addition to directories, should you
need to (a directory is just another file in Unix).
As usual, more information can be found by consulting the man pages
for the appropriate command for your platform. The commands for other
flavors of Unix will be added as needed.
In order to have these tricks persist, to survive a system reboot, the
/etc/fstab (or /etc/vfstab) file may need to have these mounts added.
Consult your local fstab(5) (or vfstab(4) for Solaris) man pages for