i knew, when this came out a few months back, that
i had put up a similar interface a long time ago, but
i couldn't track it down quickly, so i forgot about it...

but i just stumbled across it again.  it was april 2010:

>   http://lists.pglaf.org/mailman/private/gutvol-d/2010-April/date.html

the subject is "a horizontal proofing interface, line by line",
and the first post in the thread was made on april 1, 2010:

>   http://lists.pglaf.org/mailman/private/gutvol-d/2010-April/006389.html

the demo itself is here:

>   http://z-m-l.com/go/lines/pagelines.html

this is more the "smoothreading" interface, where
a person just clicks a button if a line has a mistake;
an administrator gets the report and fixes the error.

-bowerbird