[Revan]

The profile hack you are referring to is something different, because it is actually possible to detect whether or not an user has filled out said field.
There COULD be, however, a couple of column added to the user table, one for 'lastreadthread' and one for 'didreply'.
Whenever someone read a thread, they would get that threadid inserted in #1, and if they replied, they would get the same threadid inserted into #2.
So a check could be made whether or not the two are equal, if not then redirect to the thread in question.
Also this code could be placed in global.php to ensure users couldn't escape.
Also, to avoid infinite redirection loop, a check would be made against whether or not the script is showthread.php AND $_GET['threadid'] (or whatever) === $bbuserinfo['lastreadthread']

Of course this is just me thinking out loud.