Would it not be easier to restrict the size of hand luggage to bags that fit under the seat only on the lowest SH economy fares. From an IT point of view it should be easy to implement: No front row seating, no emergency row seating, under seat back packs only. When at the airport, they can put the tags they have on the bags of passengers with these restrictions, so crew can easily see any luggage in the overhead bins that should be under seats.