I have seen it happen often at DAL where the SUVs drive right up to the plane and let security conscious folks board first. I must say that I have never seen a delay of more than 5 minutes.
On the first flight of the day it is easy but on later flights the FA use placards to block off seats in the front.
Not sure how the FA would block off enough seats in WPB, where a third of the flight pax seems to need special accommodation.