I don't know if Jonathan will add this feature or not, but I can offer a work-around (this is what I do). Download each completed article to your computer. As you use them, move them to a folder that's named something like "Published Articles." That way, I know which articles I've used, and which I still need to use.
When I download them, I save them in folders that are dated by the dates that each article was completed, so I know which articles I've already downloaded from the site. Here is an example of the folder structure I use:
C:/Documents/Jiffy Articles/2011-11-13/Waiting To Be Published/Published
C:/Documents/Jiffy Articles/2011-11-21/Waiting To Be Published/Published
C:/Documents/Jiffy Articles/2011-12-05/Waiting To Be Published/Published
Any articles inside "Waiting To Be Published" have not yet been used. Any articles in "Published" have already been used. I hope this makes sense.