The problem is with Deja, not with Duplicity (Why this option is not displayed in the UI, one can only wonder).
Thus you can call Duplicity manually via: duplicity --no-encryption --gio --tempdir /var/tmp file:///media/truecrypt1 file:///home/username/backup
So you can have your cake (tmpfs) and eat (backup) it, too ;)
The problem is with Deja, not with Duplicity (Why this option is not displayed in the UI, one can only wonder). /media/ truecrypt1 file:// /home/username/ backup
Thus you can call Duplicity manually via: duplicity --no-encryption --gio --tempdir /var/tmp file://
So you can have your cake (tmpfs) and eat (backup) it, too ;)