Fully agree, many multi-storey carparks (either with paper ticket or a chip-coin) use ANPR & many are on Skidata systems. Luton Airport use Skidata and if you've pre-booked parking, the barrier just issues you a ticket on arrival with your number plate printed on it. I remember having a "chat" with an old gentleman at East Midlands Airport who had entered the car park in front of me, but dropped his chip-coin down the drain... he then wanted my chip-coin from the barrier. Had to explain to him it would be 100% useless as linked to my number plate.
Back to Towers, the really hard bit would be how to exempt premium AP holders & hotel guests from the parking charge without forcing them to register a vehicle. I suspect the "more mature" traffic team at the AT carpark exit barriers are going nowhere fast.
Before they went onto the Skidata system, didn't they ask you to use a certain two barriers for Annual Passes, e.g. the left two / right hand two, in an attempt to reduce the number of staff needed to let AP holders out? Am pretty sure they did, but as with all signs at Towers, they are widely ignored.