The rules for half squares are found in the scenario book (Black Box) on pages 4-5. They state that figures may not set up on half squares (p.4). On page 5 it goes on to say:
"Half squares are only useable by a figure which is entering the board. A figure must pay movement point(s) when entering the board on to a half square. If the player is exiting the board with a figure no movement point(s) are spent to move off of a half square, the figure is considered off the board when it moves on to it."
So half squares are unplayable except to move onto the board. In most set-ups (all?), the half squares are at entrance points/rooms. We treat the entrance rooms as if these other half squares (see Quarterback Sneak room at top of set-up) are walls because page 5 goes on to say:
"Figures can not move off the board unless exiting the board through an appropriate exit point as defined in the scenario. A figure moving off the board through an exit point cannot return at a later time."
So since you cannot use a half square except to move onto the board and cannot exit except at defined points, in which half squares do not count for movement, half squares in rooms are unplayable.