I just saved a test document in LibreOffice to take a look at this. I tried to save it in phablet but via FileManager I found it in
I guess I can see the logic of that for security but it isn't great for users in cases like this.
I can't see an easy way to break out of the libertine jail and save wherever you want- which I guess is a feature more than a bug.
The best thing might be to try
mount --bind. It's a way to mount one folder as another. I am not so familiar with it, but it is described as a solution to a similiar problem here.
One thing to be careful of, the accepted answer to this question says that if you do mount chroot directories ". You will need to take extra care when doing so, if you wish to delete the chroot from your phone, or perform other destructive actions".
Hope that helps.