Worked kind of. Its important that you first create a folder in the user-data/xenial folder
I named it SDCARD and then the mount command is:
sudo mount -B /media/phablet/ED34-76D7/ ~/.local/share/libertine-container/user-data/xenial/SDCARD
Now i can access the data from libertine apps 
So i installed the deb from apt repos which crashes immediately on startup.
After some digging I found:
https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1711337
The above bug report and a workaround with a deb from
https://www.dropbox.com/s/ogdut56m2i0k0t9/firefox_59.0.2%2Bbuild1-0ubuntu0.16.04.3_armhf.deb?dl=0
firefox starts up, but is quite unstable....