There is such a feature.
When you press 'preview', the document is forcefully saved as draft.
I made a small plugin to view the source of the drafts via the 'admin'-page.
It is in 'alpha' and will only show the page source, not rendered page.
It doesn't work out of the box. I've only tested it on unix-like systems, and you have to edit the admin.php and getDrafts.php and
change YOURDOKUWIKIDIR to the filesystem directory of your dokuwiki directory.
You can find it at:
http://folk.uio.no/twhoffma/dokuwiki/draftlist.tar.gz
//Falkor