Well, you would need to add a column to the user table entitled "floodchecktime", which by default is say 0, meaning it uses the default one.
You would have to edit the newthread.php and newreply.php. You would need to add, around where the censor stuff check is, a check for if the user is not violating his individual floodcheck (if floodchecktime <> 0).
Not that hard, but I don't have the time for it right now
I might do it when I get back from vacation in 2 weeks. Would the tiem be set in the cp?